diff --git a/lib/codeviewer/code_displayer.dart b/lib/codeviewer/code_displayer.dart
index ff48976ad8..c8cbbb0efb 100644
--- a/lib/codeviewer/code_displayer.dart
+++ b/lib/codeviewer/code_displayer.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 typedef CodeDisplayer = TextSpan Function(BuildContext context);
diff --git a/lib/codeviewer/code_segments.dart b/lib/codeviewer/code_segments.dart
index ef98ab166c..c6c94d8b3e 100644
--- a/lib/codeviewer/code_segments.dart
+++ b/lib/codeviewer/code_segments.dart
@@ -5,6 +5,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/codeviewer/code_style.dart';
 
@@ -24,6 +26,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -339,6 +343,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -1136,6 +1142,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -2122,6 +2130,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -3056,6 +3066,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -3360,6 +3372,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -3765,6 +3779,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -3934,6 +3950,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -4107,6 +4125,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -4276,6 +4296,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -4445,6 +4467,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -4745,6 +4769,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -7115,6 +7141,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -7235,6 +7263,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -7621,6 +7651,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -8048,6 +8080,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -8187,6 +8221,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -9665,6 +9701,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -9795,6 +9833,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -11919,6 +11959,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -12126,6 +12168,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -12466,6 +12510,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -13069,6 +13115,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -13503,6 +13551,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -14852,6 +14902,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(style: codeStyle.stringStyle, text: '\u0027dart:math\u0027'),
@@ -15242,6 +15294,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -15718,6 +15772,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -16245,6 +16301,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -16506,6 +16564,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -16938,6 +16998,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(style: codeStyle.stringStyle, text: '\u0027dart:math\u0027'),
@@ -18998,6 +19060,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -22201,6 +22265,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -24018,6 +24084,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -24225,6 +24293,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -25045,6 +25115,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -25968,6 +26040,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -27075,6 +27149,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -27334,6 +27410,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -28251,6 +28329,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -28543,6 +28623,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -28945,6 +29027,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -29393,6 +29477,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -29721,6 +29807,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -30243,6 +30331,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -33130,6 +33220,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -34398,6 +34490,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -34957,6 +35051,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(style: codeStyle.stringStyle, text: '\u0027dart:math\u0027'),
@@ -35508,6 +35604,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -35850,6 +35948,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -36104,6 +36204,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -36331,6 +36433,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -37595,6 +37699,8 @@ class CodeSegments {
   static TextSpan sharedYAxisTransitionDemo(BuildContext context) {
     final codeStyle = CodeStyle.of(context);
     return TextSpan(children: [
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(style: codeStyle.stringStyle, text: '\u0027dart:math\u0027'),
@@ -38579,6 +38685,8 @@ class CodeSegments {
   static TextSpan sharedZAxisTransitionDemo(BuildContext context) {
     final codeStyle = CodeStyle.of(context);
     return TextSpan(children: [
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -39834,6 +39942,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(style: codeStyle.stringStyle, text: '\u0027dart:math\u0027'),
@@ -40419,6 +40529,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -40666,6 +40778,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -41070,6 +41184,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -41549,6 +41665,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -43956,6 +44074,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -44202,6 +44322,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -47308,6 +47430,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
@@ -47930,6 +48054,8 @@ class CodeSegments {
       TextSpan(
           style: codeStyle.commentStyle, text: '// found in the LICENSE file.'),
       TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
+      TextSpan(style: codeStyle.commentStyle, text: '// @dart=2.9'),
+      TextSpan(style: codeStyle.baseStyle, text: '\u000a\u000a'),
       TextSpan(style: codeStyle.keywordStyle, text: 'import'),
       TextSpan(style: codeStyle.baseStyle, text: ' '),
       TextSpan(
diff --git a/lib/codeviewer/code_style.dart b/lib/codeviewer/code_style.dart
index f5a6de1c31..0fb1870ec1 100644
--- a/lib/codeviewer/code_style.dart
+++ b/lib/codeviewer/code_style.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 class CodeStyle extends InheritedWidget {
diff --git a/lib/constants.dart b/lib/constants.dart
index 735135324b..775bb7acec 100644
--- a/lib/constants.dart
+++ b/lib/constants.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 // Only put constants shared between files here.
 
 // height of the 'Gallery' header
diff --git a/lib/data/demos.dart b/lib/data/demos.dart
index 0f45b93a4a..92f65dc19a 100644
--- a/lib/data/demos.dart
+++ b/lib/data/demos.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:collection';
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
diff --git a/lib/data/gallery_options.dart b/lib/data/gallery_options.dart
index 94e0cb3bca..8c071e07dd 100644
--- a/lib/data/gallery_options.dart
+++ b/lib/data/gallery_options.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 
 import 'package:flutter/material.dart';
diff --git a/lib/data/icons.dart b/lib/data/icons.dart
index ddf198f53f..c05c9a507e 100644
--- a/lib/data/icons.dart
+++ b/lib/data/icons.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 class GalleryIcons {
diff --git a/lib/deferred_widget.dart b/lib/deferred_widget.dart
index e2b426b637..bb3d389e21 100644
--- a/lib/deferred_widget.dart
+++ b/lib/deferred_widget.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'package:flutter/material.dart';
 
diff --git a/lib/demos/cupertino/cupertino_activity_indicator_demo.dart b/lib/demos/cupertino/cupertino_activity_indicator_demo.dart
index 3b4268b23e..42ab2eacae 100644
--- a/lib/demos/cupertino/cupertino_activity_indicator_demo.dart
+++ b/lib/demos/cupertino/cupertino_activity_indicator_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/cupertino/cupertino_alert_demo.dart b/lib/demos/cupertino/cupertino_alert_demo.dart
index 1a7f69cfc2..34ef1633b1 100644
--- a/lib/demos/cupertino/cupertino_alert_demo.dart
+++ b/lib/demos/cupertino/cupertino_alert_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/cupertino/cupertino_button_demo.dart b/lib/demos/cupertino/cupertino_button_demo.dart
index 93160677c5..38c881ed05 100644
--- a/lib/demos/cupertino/cupertino_button_demo.dart
+++ b/lib/demos/cupertino/cupertino_button_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/cupertino/cupertino_context_menu_demo.dart b/lib/demos/cupertino/cupertino_context_menu_demo.dart
index 2c34d45512..3a9ec64112 100644
--- a/lib/demos/cupertino/cupertino_context_menu_demo.dart
+++ b/lib/demos/cupertino/cupertino_context_menu_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/cupertino/cupertino_demos.dart b/lib/demos/cupertino/cupertino_demos.dart
index 521e28f982..9b68dc8bc4 100644
--- a/lib/demos/cupertino/cupertino_demos.dart
+++ b/lib/demos/cupertino/cupertino_demos.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 export 'package:gallery/demos/cupertino/cupertino_activity_indicator_demo.dart';
 export 'package:gallery/demos/cupertino/cupertino_alert_demo.dart';
 export 'package:gallery/demos/cupertino/cupertino_button_demo.dart';
diff --git a/lib/demos/cupertino/cupertino_navigation_bar_demo.dart b/lib/demos/cupertino/cupertino_navigation_bar_demo.dart
index a45740797c..74e1dd4393 100644
--- a/lib/demos/cupertino/cupertino_navigation_bar_demo.dart
+++ b/lib/demos/cupertino/cupertino_navigation_bar_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/cupertino/cupertino_picker_demo.dart b/lib/demos/cupertino/cupertino_picker_demo.dart
index 9446310b61..57f6ec4db2 100644
--- a/lib/demos/cupertino/cupertino_picker_demo.dart
+++ b/lib/demos/cupertino/cupertino_picker_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:intl/intl.dart';
diff --git a/lib/demos/cupertino/cupertino_refresh_demo.dart b/lib/demos/cupertino/cupertino_refresh_demo.dart
index 91ae03ebc7..6720bb038c 100644
--- a/lib/demos/cupertino/cupertino_refresh_demo.dart
+++ b/lib/demos/cupertino/cupertino_refresh_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math' show Random;
 
 import 'package:flutter/cupertino.dart';
diff --git a/lib/demos/cupertino/cupertino_segmented_control_demo.dart b/lib/demos/cupertino/cupertino_segmented_control_demo.dart
index 67b11ffc7a..100e49ad47 100644
--- a/lib/demos/cupertino/cupertino_segmented_control_demo.dart
+++ b/lib/demos/cupertino/cupertino_segmented_control_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/cupertino/cupertino_slider_demo.dart b/lib/demos/cupertino/cupertino_slider_demo.dart
index 86a389f010..9b4354c630 100644
--- a/lib/demos/cupertino/cupertino_slider_demo.dart
+++ b/lib/demos/cupertino/cupertino_slider_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/cupertino/cupertino_switch_demo.dart b/lib/demos/cupertino/cupertino_switch_demo.dart
index 78fadf52d4..885bf58a81 100644
--- a/lib/demos/cupertino/cupertino_switch_demo.dart
+++ b/lib/demos/cupertino/cupertino_switch_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/cupertino/cupertino_tab_bar_demo.dart b/lib/demos/cupertino/cupertino_tab_bar_demo.dart
index 4cd637e801..8a9c348fd9 100644
--- a/lib/demos/cupertino/cupertino_tab_bar_demo.dart
+++ b/lib/demos/cupertino/cupertino_tab_bar_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/cupertino/cupertino_text_field_demo.dart b/lib/demos/cupertino/cupertino_text_field_demo.dart
index b6967e12f2..067e4d7c23 100644
--- a/lib/demos/cupertino/cupertino_text_field_demo.dart
+++ b/lib/demos/cupertino/cupertino_text_field_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/cupertino/demo_types.dart b/lib/demos/cupertino/demo_types.dart
index 6b07491c3f..d5798f6054 100644
--- a/lib/demos/cupertino/demo_types.dart
+++ b/lib/demos/cupertino/demo_types.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 enum AlertDemoType {
   alert,
   alertTitle,
diff --git a/lib/demos/material/app_bar_demo.dart b/lib/demos/material/app_bar_demo.dart
index 54932e34d9..217b95518d 100644
--- a/lib/demos/material/app_bar_demo.dart
+++ b/lib/demos/material/app_bar_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/material/banner_demo.dart b/lib/demos/material/banner_demo.dart
index a816a4c313..13650f9798 100644
--- a/lib/demos/material/banner_demo.dart
+++ b/lib/demos/material/banner_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/material/bottom_app_bar_demo.dart b/lib/demos/material/bottom_app_bar_demo.dart
index f827dc95ec..e8c31a639e 100644
--- a/lib/demos/material/bottom_app_bar_demo.dart
+++ b/lib/demos/material/bottom_app_bar_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/material/bottom_navigation_demo.dart b/lib/demos/material/bottom_navigation_demo.dart
index e1a94e63e9..d69094849c 100644
--- a/lib/demos/material/bottom_navigation_demo.dart
+++ b/lib/demos/material/bottom_navigation_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/material/bottom_sheet_demo.dart b/lib/demos/material/bottom_sheet_demo.dart
index 1756f41b41..25bb4685a8 100644
--- a/lib/demos/material/bottom_sheet_demo.dart
+++ b/lib/demos/material/bottom_sheet_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/material/button_demo.dart b/lib/demos/material/button_demo.dart
index 01a7e3ed8d..4f8b6e3464 100644
--- a/lib/demos/material/button_demo.dart
+++ b/lib/demos/material/button_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/material/cards_demo.dart b/lib/demos/material/cards_demo.dart
index eba3c58470..05539fa1c2 100644
--- a/lib/demos/material/cards_demo.dart
+++ b/lib/demos/material/cards_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/material/chip_demo.dart b/lib/demos/material/chip_demo.dart
index 0b567bbc33..8a1f3ff622 100644
--- a/lib/demos/material/chip_demo.dart
+++ b/lib/demos/material/chip_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/demos/material/material_demo_types.dart';
diff --git a/lib/demos/material/data_table_demo.dart b/lib/demos/material/data_table_demo.dart
index 30042cee35..51c52209ab 100644
--- a/lib/demos/material/data_table_demo.dart
+++ b/lib/demos/material/data_table_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/data/gallery_options.dart';
diff --git a/lib/demos/material/dialog_demo.dart b/lib/demos/material/dialog_demo.dart
index 1c9ca1efe9..86e29c6475 100644
--- a/lib/demos/material/dialog_demo.dart
+++ b/lib/demos/material/dialog_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/material/divider_demo.dart b/lib/demos/material/divider_demo.dart
index a95efbc023..182a12b93b 100644
--- a/lib/demos/material/divider_demo.dart
+++ b/lib/demos/material/divider_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/demos/material/material_demo_types.dart';
diff --git a/lib/demos/material/grid_list_demo.dart b/lib/demos/material/grid_list_demo.dart
index ed2975255a..1ebdacafaa 100644
--- a/lib/demos/material/grid_list_demo.dart
+++ b/lib/demos/material/grid_list_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/demos/material/material_demo_types.dart';
diff --git a/lib/demos/material/list_demo.dart b/lib/demos/material/list_demo.dart
index c252ffae43..a42f0c224b 100644
--- a/lib/demos/material/list_demo.dart
+++ b/lib/demos/material/list_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/material/material_demo_types.dart b/lib/demos/material/material_demo_types.dart
index 88c92d43f3..cc70bed62c 100644
--- a/lib/demos/material/material_demo_types.dart
+++ b/lib/demos/material/material_demo_types.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 enum BottomNavigationDemoType {
   withLabels,
   withoutLabels,
diff --git a/lib/demos/material/material_demos.dart b/lib/demos/material/material_demos.dart
index 7239da9204..4a4582535a 100644
--- a/lib/demos/material/material_demos.dart
+++ b/lib/demos/material/material_demos.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 export 'package:gallery/demos/material/app_bar_demo.dart';
 export 'package:gallery/demos/material/banner_demo.dart';
 export 'package:gallery/demos/material/bottom_app_bar_demo.dart';
diff --git a/lib/demos/material/menu_demo.dart b/lib/demos/material/menu_demo.dart
index 1486578c08..0fb4949ee0 100644
--- a/lib/demos/material/menu_demo.dart
+++ b/lib/demos/material/menu_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/material/navigation_drawer.dart b/lib/demos/material/navigation_drawer.dart
index 9db4d73c11..598a61d91a 100644
--- a/lib/demos/material/navigation_drawer.dart
+++ b/lib/demos/material/navigation_drawer.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/material/navigation_rail_demo.dart b/lib/demos/material/navigation_rail_demo.dart
index f1fd3eaa0b..da47b21237 100644
--- a/lib/demos/material/navigation_rail_demo.dart
+++ b/lib/demos/material/navigation_rail_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/material/picker_demo.dart b/lib/demos/material/picker_demo.dart
index 75ab6d4f04..cb459ffb95 100644
--- a/lib/demos/material/picker_demo.dart
+++ b/lib/demos/material/picker_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/demos/material/material_demo_types.dart';
diff --git a/lib/demos/material/progress_indicator_demo.dart b/lib/demos/material/progress_indicator_demo.dart
index 2efc63c269..9efad0d4d6 100644
--- a/lib/demos/material/progress_indicator_demo.dart
+++ b/lib/demos/material/progress_indicator_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/demos/material/material_demo_types.dart';
diff --git a/lib/demos/material/selection_controls_demo.dart b/lib/demos/material/selection_controls_demo.dart
index 000ca44d90..b2d8425d37 100644
--- a/lib/demos/material/selection_controls_demo.dart
+++ b/lib/demos/material/selection_controls_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/material/sliders_demo.dart b/lib/demos/material/sliders_demo.dart
index d7c69aff23..d817e3e4e9 100644
--- a/lib/demos/material/sliders_demo.dart
+++ b/lib/demos/material/sliders_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math' as math;
 
 import 'package:flutter/material.dart';
diff --git a/lib/demos/material/snackbar_demo.dart b/lib/demos/material/snackbar_demo.dart
index a19d2da097..a69566ecd8 100644
--- a/lib/demos/material/snackbar_demo.dart
+++ b/lib/demos/material/snackbar_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/material/tabs_demo.dart b/lib/demos/material/tabs_demo.dart
index b6c9472f98..ebeb2e93fc 100644
--- a/lib/demos/material/tabs_demo.dart
+++ b/lib/demos/material/tabs_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/demos/material/material_demo_types.dart';
diff --git a/lib/demos/material/text_field_demo.dart b/lib/demos/material/text_field_demo.dart
index 49939f351d..d2df2fc9fc 100644
--- a/lib/demos/material/text_field_demo.dart
+++ b/lib/demos/material/text_field_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 
diff --git a/lib/demos/material/tooltip_demo.dart b/lib/demos/material/tooltip_demo.dart
index 96684b9b1c..e439b25e5a 100644
--- a/lib/demos/material/tooltip_demo.dart
+++ b/lib/demos/material/tooltip_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/reference/colors_demo.dart b/lib/demos/reference/colors_demo.dart
index c78200cbf6..9211287969 100644
--- a/lib/demos/reference/colors_demo.dart
+++ b/lib/demos/reference/colors_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/reference/motion_demo_container_transition.dart b/lib/demos/reference/motion_demo_container_transition.dart
index 5c98c88aa2..99efc011ec 100644
--- a/lib/demos/reference/motion_demo_container_transition.dart
+++ b/lib/demos/reference/motion_demo_container_transition.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/reference/motion_demo_fade_scale_transition.dart b/lib/demos/reference/motion_demo_fade_scale_transition.dart
index 2c06191856..9f775856ef 100644
--- a/lib/demos/reference/motion_demo_fade_scale_transition.dart
+++ b/lib/demos/reference/motion_demo_fade_scale_transition.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/reference/motion_demo_fade_through_transition.dart b/lib/demos/reference/motion_demo_fade_through_transition.dart
index 27978238e0..024625ea18 100644
--- a/lib/demos/reference/motion_demo_fade_through_transition.dart
+++ b/lib/demos/reference/motion_demo_fade_through_transition.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/reference/motion_demo_shared_x_axis_transition.dart b/lib/demos/reference/motion_demo_shared_x_axis_transition.dart
index bc56187db3..86e273809c 100644
--- a/lib/demos/reference/motion_demo_shared_x_axis_transition.dart
+++ b/lib/demos/reference/motion_demo_shared_x_axis_transition.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/reference/motion_demo_shared_y_axis_transition.dart b/lib/demos/reference/motion_demo_shared_y_axis_transition.dart
index 95307df70a..9bc4149afb 100644
--- a/lib/demos/reference/motion_demo_shared_y_axis_transition.dart
+++ b/lib/demos/reference/motion_demo_shared_y_axis_transition.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'dart:math';
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
diff --git a/lib/demos/reference/motion_demo_shared_z_axis_transition.dart b/lib/demos/reference/motion_demo_shared_z_axis_transition.dart
index eb3274f5c6..40b4ecffd3 100644
--- a/lib/demos/reference/motion_demo_shared_z_axis_transition.dart
+++ b/lib/demos/reference/motion_demo_shared_z_axis_transition.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/demos/reference/transformations_demo.dart b/lib/demos/reference/transformations_demo.dart
index 3aa3bc3806..49f20adf2f 100644
--- a/lib/demos/reference/transformations_demo.dart
+++ b/lib/demos/reference/transformations_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/demos/reference/transformations_demo_board.dart b/lib/demos/reference/transformations_demo_board.dart
index aa48ae34b9..387d36320d 100644
--- a/lib/demos/reference/transformations_demo_board.dart
+++ b/lib/demos/reference/transformations_demo_board.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:collection' show IterableMixin;
 import 'dart:math';
 import 'dart:ui' show Vertices;
diff --git a/lib/demos/reference/transformations_demo_color_picker.dart b/lib/demos/reference/transformations_demo_color_picker.dart
index b520c96ff8..53cafae4e8 100644
--- a/lib/demos/reference/transformations_demo_color_picker.dart
+++ b/lib/demos/reference/transformations_demo_color_picker.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 // A generic widget for a list of selectable colors.
diff --git a/lib/demos/reference/transformations_demo_edit_board_point.dart b/lib/demos/reference/transformations_demo_edit_board_point.dart
index 3011553be4..a35ba62e22 100644
--- a/lib/demos/reference/transformations_demo_edit_board_point.dart
+++ b/lib/demos/reference/transformations_demo_edit_board_point.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/themes/gallery_theme_data.dart';
 import 'transformations_demo_board.dart';
diff --git a/lib/demos/reference/typography_demo.dart b/lib/demos/reference/typography_demo.dart
index 9adebcd014..24a0cef166 100644
--- a/lib/demos/reference/typography_demo.dart
+++ b/lib/demos/reference/typography_demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/feature_discovery/animation.dart b/lib/feature_discovery/animation.dart
index f8b62bf5d2..a97960255b 100644
--- a/lib/feature_discovery/animation.dart
+++ b/lib/feature_discovery/animation.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 /// Animations class to compute animation values for overlay widgets.
diff --git a/lib/feature_discovery/feature_discovery.dart b/lib/feature_discovery/feature_discovery.dart
index f608b1cd6d..cd8ddc5317 100644
--- a/lib/feature_discovery/feature_discovery.dart
+++ b/lib/feature_discovery/feature_discovery.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart';
 
diff --git a/lib/feature_discovery/overlay.dart b/lib/feature_discovery/overlay.dart
index 7bba12d688..e093f16210 100644
--- a/lib/feature_discovery/overlay.dart
+++ b/lib/feature_discovery/overlay.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';
diff --git a/lib/layout/highlight_focus.dart b/lib/layout/highlight_focus.dart
index a42a3ee3cf..15abb6369d 100644
--- a/lib/layout/highlight_focus.dart
+++ b/lib/layout/highlight_focus.dart
@@ -10,9 +10,9 @@ import 'package:flutter/services.dart';
 /// Wrap your widget as [child] of a [HighlightFocus] widget.
 class HighlightFocus extends StatefulWidget {
   const HighlightFocus({
-    Key key,
-    @required this.onPressed,
-    @required this.child,
+    Key? key,
+    required this.onPressed,
+    required this.child,
     this.highlightColor,
     this.borderColor,
     this.hasFocus = true,
@@ -30,23 +30,23 @@ class HighlightFocus extends StatefulWidget {
   /// is focused.
   /// Use [Colors.transparent] if you do not want one.
   /// Use an opacity less than 1 to make the underlying widget visible.
-  final Color highlightColor;
+  final Color? highlightColor;
 
   /// [borderColor] is the color of the border when the widget is focused.
-  final Color borderColor;
+  final Color? borderColor;
 
   /// [hasFocus] is true when focusing on the widget is allowed.
   /// Set to false if you want the child to skip focus.
   final bool hasFocus;
 
-  final String debugLabel;
+  final String? debugLabel;
 
   @override
   _HighlightFocusState createState() => _HighlightFocusState();
 }
 
 class _HighlightFocusState extends State<HighlightFocus> {
-  bool isFocused;
+  late bool isFocused;
 
   @override
   void initState() {
diff --git a/lib/layout/image_placeholder.dart b/lib/layout/image_placeholder.dart
index 682ef9d1a1..5a9c38e0a1 100644
--- a/lib/layout/image_placeholder.dart
+++ b/lib/layout/image_placeholder.dart
@@ -12,18 +12,16 @@ import 'package:flutter/material.dart';
 /// the image with another widget, for example an [Ink.image].
 class FadeInImagePlaceholder extends StatelessWidget {
   const FadeInImagePlaceholder({
-    Key key,
-    @required this.image,
-    @required this.placeholder,
+    Key? key,
+    required this.image,
+    required this.placeholder,
     this.child,
     this.duration = const Duration(milliseconds: 500),
     this.excludeFromSemantics = false,
     this.width,
     this.height,
     this.fit,
-  })  : assert(placeholder != null),
-        assert(image != null),
-        super(key: key);
+  }) : super(key: key);
 
   /// The target image that we are loading into memory.
   final ImageProvider image;
@@ -35,7 +33,7 @@ class FadeInImagePlaceholder extends StatelessWidget {
   /// loaded.
   ///
   /// Defaults to display the [image].
-  final Widget child;
+  final Widget? child;
 
   /// The duration for how long the fade out of the placeholder and
   /// fade in of [child] should take.
@@ -45,13 +43,13 @@ class FadeInImagePlaceholder extends StatelessWidget {
   final bool excludeFromSemantics;
 
   /// See [Image.width].
-  final double width;
+  final double? width;
 
   /// See [Image.height].
-  final double height;
+  final double? height;
 
   /// See [Image.fit].
-  final BoxFit fit;
+  final BoxFit? fit;
 
   @override
   Widget build(BuildContext context) {
diff --git a/lib/layout/text_scale.dart b/lib/layout/text_scale.dart
index 1e159510b5..b45afd5109 100644
--- a/lib/layout/text_scale.dart
+++ b/lib/layout/text_scale.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';
diff --git a/lib/main.dart b/lib/main.dart
index f2edcb7bfd..2203ae6c2e 100644
--- a/lib/main.dart
+++ b/lib/main.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/scheduler.dart' show timeDilation;
diff --git a/lib/pages/about.dart b/lib/pages/about.dart
index f9dc786a4b..4fb988531d 100644
--- a/lib/pages/about.dart
+++ b/lib/pages/about.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/pages/backdrop.dart b/lib/pages/backdrop.dart
index 832da4ec77..fd050bd6ff 100644
--- a/lib/pages/backdrop.dart
+++ b/lib/pages/backdrop.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter/services.dart';
diff --git a/lib/pages/category_list_item.dart b/lib/pages/category_list_item.dart
index 1369d29eb0..c4cb4b432e 100644
--- a/lib/pages/category_list_item.dart
+++ b/lib/pages/category_list_item.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/data/demos.dart';
diff --git a/lib/pages/demo.dart b/lib/pages/demo.dart
index b87a00a3db..a169b57a91 100644
--- a/lib/pages/demo.dart
+++ b/lib/pages/demo.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:io' show Platform;
 
 import 'package:flutter/cupertino.dart';
diff --git a/lib/pages/home.dart b/lib/pages/home.dart
index 196c04c42f..678c725623 100644
--- a/lib/pages/home.dart
+++ b/lib/pages/home.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:math' as math;
 
diff --git a/lib/pages/settings.dart b/lib/pages/settings.dart
index 43ecb0bb9a..53ce93a84f 100644
--- a/lib/pages/settings.dart
+++ b/lib/pages/settings.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:collection';
 
 import 'package:collection/collection.dart';
diff --git a/lib/pages/settings_icon/icon.dart b/lib/pages/settings_icon/icon.dart
index 9a00d88f59..9072a921d7 100644
--- a/lib/pages/settings_icon/icon.dart
+++ b/lib/pages/settings_icon/icon.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 import 'package:flutter/material.dart';
 import 'package:gallery/pages/settings_icon/metrics.dart';
diff --git a/lib/pages/settings_icon/metrics.dart b/lib/pages/settings_icon/metrics.dart
index 2d3679d5e6..2cc210245a 100644
--- a/lib/pages/settings_icon/metrics.dart
+++ b/lib/pages/settings_icon/metrics.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 import 'package:flutter/material.dart';
 
diff --git a/lib/pages/settings_list_item.dart b/lib/pages/settings_list_item.dart
index 2d8920c05a..664729e5f1 100644
--- a/lib/pages/settings_list_item.dart
+++ b/lib/pages/settings_list_item.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:collection';
 
 import 'package:flutter/material.dart';
diff --git a/lib/pages/splash.dart b/lib/pages/splash.dart
index 56def49950..8e655f54b5 100644
--- a/lib/pages/splash.dart
+++ b/lib/pages/splash.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';
diff --git a/lib/routes.dart b/lib/routes.dart
index 2f1cc8d9ad..80cc258e43 100644
--- a/lib/routes.dart
+++ b/lib/routes.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:gallery/deferred_widget.dart';
diff --git a/lib/studies/crane/app.dart b/lib/studies/crane/app.dart
index e666a6dfe8..adb54d0760 100644
--- a/lib/studies/crane/app.dart
+++ b/lib/studies/crane/app.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/data/gallery_options.dart';
diff --git a/lib/studies/crane/backdrop.dart b/lib/studies/crane/backdrop.dart
index f486005a96..53632f85cf 100644
--- a/lib/studies/crane/backdrop.dart
+++ b/lib/studies/crane/backdrop.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/crane/backlayer.dart b/lib/studies/crane/backlayer.dart
index d4e1ba3133..2b631ed266 100644
--- a/lib/studies/crane/backlayer.dart
+++ b/lib/studies/crane/backlayer.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 abstract class BackLayerItem extends StatefulWidget {
diff --git a/lib/studies/crane/border_tab_indicator.dart b/lib/studies/crane/border_tab_indicator.dart
index 13fd9ced09..82b0fe135b 100644
--- a/lib/studies/crane/border_tab_indicator.dart
+++ b/lib/studies/crane/border_tab_indicator.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 class BorderTabIndicator extends Decoration {
diff --git a/lib/studies/crane/colors.dart b/lib/studies/crane/colors.dart
index 2550cfa45b..aa021c86a4 100644
--- a/lib/studies/crane/colors.dart
+++ b/lib/studies/crane/colors.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 const cranePurple700 = Color(0xFF720D5D);
diff --git a/lib/studies/crane/eat_form.dart b/lib/studies/crane/eat_form.dart
index cfc346b007..eebdf72b9b 100644
--- a/lib/studies/crane/eat_form.dart
+++ b/lib/studies/crane/eat_form.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/crane/fly_form.dart b/lib/studies/crane/fly_form.dart
index 4a4d3ec2fe..bd20c22d5e 100644
--- a/lib/studies/crane/fly_form.dart
+++ b/lib/studies/crane/fly_form.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/crane/header_form.dart b/lib/studies/crane/header_form.dart
index 8fc633fe47..59c67010dc 100644
--- a/lib/studies/crane/header_form.dart
+++ b/lib/studies/crane/header_form.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/layout/adaptive.dart';
 import 'package:gallery/studies/crane/colors.dart';
diff --git a/lib/studies/crane/item_cards.dart b/lib/studies/crane/item_cards.dart
index 7d62d7c43f..128b106d61 100644
--- a/lib/studies/crane/item_cards.dart
+++ b/lib/studies/crane/item_cards.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:gallery/layout/adaptive.dart';
diff --git a/lib/studies/crane/model/data.dart b/lib/studies/crane/model/data.dart
index d6a874212f..94181ed22e 100644
--- a/lib/studies/crane/model/data.dart
+++ b/lib/studies/crane/model/data.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/studies/crane/model/destination.dart';
diff --git a/lib/studies/crane/model/destination.dart b/lib/studies/crane/model/destination.dart
index ee66a9eb7b..5c9dba5be7 100644
--- a/lib/studies/crane/model/destination.dart
+++ b/lib/studies/crane/model/destination.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/studies/crane/model/formatters.dart b/lib/studies/crane/model/formatters.dart
index f5998d2bfc..87bcb8ec6a 100644
--- a/lib/studies/crane/model/formatters.dart
+++ b/lib/studies/crane/model/formatters.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/studies/crane/routes.dart b/lib/studies/crane/routes.dart
index 9d0d6bc595..ffe7d47e35 100644
--- a/lib/studies/crane/routes.dart
+++ b/lib/studies/crane/routes.dart
@@ -2,4 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 const String defaultRoute = '/crane';
diff --git a/lib/studies/crane/sleep_form.dart b/lib/studies/crane/sleep_form.dart
index dfcf47d2f6..0575fba1de 100644
--- a/lib/studies/crane/sleep_form.dart
+++ b/lib/studies/crane/sleep_form.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/studies/crane/theme.dart b/lib/studies/crane/theme.dart
index 88d21ea5ae..ab35cf27ed 100644
--- a/lib/studies/crane/theme.dart
+++ b/lib/studies/crane/theme.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/layout/letter_spacing.dart';
 import 'package:gallery/studies/crane/colors.dart';
diff --git a/lib/studies/fortnightly/app.dart b/lib/studies/fortnightly/app.dart
index 92d9e7e690..69358b6f71 100644
--- a/lib/studies/fortnightly/app.dart
+++ b/lib/studies/fortnightly/app.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/fortnightly/routes.dart b/lib/studies/fortnightly/routes.dart
index 3005294ff6..96c14ac522 100644
--- a/lib/studies/fortnightly/routes.dart
+++ b/lib/studies/fortnightly/routes.dart
@@ -2,4 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 const String defaultRoute = '/fortnightly';
diff --git a/lib/studies/fortnightly/shared.dart b/lib/studies/fortnightly/shared.dart
index d5bd408b0b..a8565ad6ad 100644
--- a/lib/studies/fortnightly/shared.dart
+++ b/lib/studies/fortnightly/shared.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 
diff --git a/lib/studies/rally/app.dart b/lib/studies/rally/app.dart
index 86d965e9fd..e8a42026cf 100644
--- a/lib/studies/rally/app.dart
+++ b/lib/studies/rally/app.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
diff --git a/lib/studies/rally/charts/line_chart.dart b/lib/studies/rally/charts/line_chart.dart
index a054250d2c..5344b06b6f 100644
--- a/lib/studies/rally/charts/line_chart.dart
+++ b/lib/studies/rally/charts/line_chart.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/semantics.dart';
 
diff --git a/lib/studies/rally/charts/pie_chart.dart b/lib/studies/rally/charts/pie_chart.dart
index 9621962a79..79cf672f6e 100644
--- a/lib/studies/rally/charts/pie_chart.dart
+++ b/lib/studies/rally/charts/pie_chart.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math' as math;
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/rally/charts/vertical_fraction_bar.dart b/lib/studies/rally/charts/vertical_fraction_bar.dart
index 31a5ff4eed..d7d987af47 100644
--- a/lib/studies/rally/charts/vertical_fraction_bar.dart
+++ b/lib/studies/rally/charts/vertical_fraction_bar.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 class VerticalFractionBar extends StatelessWidget {
diff --git a/lib/studies/rally/colors.dart b/lib/studies/rally/colors.dart
index eb13c219e5..eb855b3ebe 100644
--- a/lib/studies/rally/colors.dart
+++ b/lib/studies/rally/colors.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:ui';
 
 /// Most color assignments in Rally are not like the typical color
diff --git a/lib/studies/rally/data.dart b/lib/studies/rally/data.dart
index 07b3ca01a1..ce92c7a9cb 100644
--- a/lib/studies/rally/data.dart
+++ b/lib/studies/rally/data.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/rally/finance.dart b/lib/studies/rally/finance.dart
index f1a5048e92..360c1021c6 100644
--- a/lib/studies/rally/finance.dart
+++ b/lib/studies/rally/finance.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math' as math;
 
 import 'package:animations/animations.dart';
diff --git a/lib/studies/rally/formatters.dart b/lib/studies/rally/formatters.dart
index ca38d3792b..6f1fc8992b 100644
--- a/lib/studies/rally/formatters.dart
+++ b/lib/studies/rally/formatters.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/data/gallery_options.dart';
 import 'package:intl/intl.dart';
diff --git a/lib/studies/rally/home.dart b/lib/studies/rally/home.dart
index 0987b1a8d5..b8d2ad5d8e 100644
--- a/lib/studies/rally/home.dart
+++ b/lib/studies/rally/home.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/data/gallery_options.dart';
diff --git a/lib/studies/rally/login.dart b/lib/studies/rally/login.dart
index 44fca892d9..70e8d7fcab 100644
--- a/lib/studies/rally/login.dart
+++ b/lib/studies/rally/login.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/rally/routes.dart b/lib/studies/rally/routes.dart
index 5e89d93c32..378c67f855 100644
--- a/lib/studies/rally/routes.dart
+++ b/lib/studies/rally/routes.dart
@@ -2,5 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 const String loginRoute = '/rally/login';
 const String homeRoute = '/rally';
diff --git a/lib/studies/rally/tabs/accounts.dart b/lib/studies/rally/tabs/accounts.dart
index 734d0e473e..749de0527e 100644
--- a/lib/studies/rally/tabs/accounts.dart
+++ b/lib/studies/rally/tabs/accounts.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/rally/tabs/bills.dart b/lib/studies/rally/tabs/bills.dart
index c22794cd7e..4c369f6122 100644
--- a/lib/studies/rally/tabs/bills.dart
+++ b/lib/studies/rally/tabs/bills.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/widgets.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/rally/tabs/budgets.dart b/lib/studies/rally/tabs/budgets.dart
index f228299779..b2d1c5b78f 100644
--- a/lib/studies/rally/tabs/budgets.dart
+++ b/lib/studies/rally/tabs/budgets.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/widgets.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/rally/tabs/overview.dart b/lib/studies/rally/tabs/overview.dart
index 8b245463c2..2f2c044e71 100644
--- a/lib/studies/rally/tabs/overview.dart
+++ b/lib/studies/rally/tabs/overview.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math' as math;
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/rally/tabs/settings.dart b/lib/studies/rally/tabs/settings.dart
index 66f213bce5..c49def03d1 100644
--- a/lib/studies/rally/tabs/settings.dart
+++ b/lib/studies/rally/tabs/settings.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:gallery/layout/adaptive.dart';
diff --git a/lib/studies/rally/tabs/sidebar.dart b/lib/studies/rally/tabs/sidebar.dart
index 37b275f6af..a88014725c 100644
--- a/lib/studies/rally/tabs/sidebar.dart
+++ b/lib/studies/rally/tabs/sidebar.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/layout/adaptive.dart';
 import 'package:gallery/studies/rally/colors.dart';
diff --git a/lib/studies/reply/adaptive_nav.dart b/lib/studies/reply/adaptive_nav.dart
index 51f40abb29..17579836a3 100644
--- a/lib/studies/reply/adaptive_nav.dart
+++ b/lib/studies/reply/adaptive_nav.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'dart:math' as math;
 import 'dart:ui' as ui;
 
diff --git a/lib/studies/reply/app.dart b/lib/studies/reply/app.dart
index d70acc628d..3bb119dbd6 100644
--- a/lib/studies/reply/app.dart
+++ b/lib/studies/reply/app.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/reply/bottom_drawer.dart b/lib/studies/reply/bottom_drawer.dart
index fc9fbf8dd1..0702524419 100644
--- a/lib/studies/reply/bottom_drawer.dart
+++ b/lib/studies/reply/bottom_drawer.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/studies/reply/colors.dart';
 
diff --git a/lib/studies/reply/colors.dart b/lib/studies/reply/colors.dart
index 3c0310f394..8c7babd75a 100644
--- a/lib/studies/reply/colors.dart
+++ b/lib/studies/reply/colors.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 class ReplyColors {
diff --git a/lib/studies/reply/compose_page.dart b/lib/studies/reply/compose_page.dart
index f8f8a77797..27a1b572c6 100644
--- a/lib/studies/reply/compose_page.dart
+++ b/lib/studies/reply/compose_page.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/studies/reply/model/email_store.dart';
 import 'package:provider/provider.dart';
diff --git a/lib/studies/reply/mail_card_preview.dart b/lib/studies/reply/mail_card_preview.dart
index 8806f463c6..5889a57b37 100644
--- a/lib/studies/reply/mail_card_preview.dart
+++ b/lib/studies/reply/mail_card_preview.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:animations/animations.dart';
 import 'package:flutter/material.dart';
 import 'package:gallery/layout/adaptive.dart';
diff --git a/lib/studies/reply/mail_view_page.dart b/lib/studies/reply/mail_view_page.dart
index 2b7ad73933..10d42f509f 100644
--- a/lib/studies/reply/mail_view_page.dart
+++ b/lib/studies/reply/mail_view_page.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/studies/reply/model/email_model.dart';
 import 'package:gallery/studies/reply/model/email_store.dart';
diff --git a/lib/studies/reply/mailbox_body.dart b/lib/studies/reply/mailbox_body.dart
index 337e61ed75..0212ab3144 100644
--- a/lib/studies/reply/mailbox_body.dart
+++ b/lib/studies/reply/mailbox_body.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/layout/adaptive.dart';
 import 'package:gallery/studies/reply/mail_card_preview.dart';
diff --git a/lib/studies/reply/model/email_model.dart b/lib/studies/reply/model/email_model.dart
index d6acc92953..ce7c4d97ea 100644
--- a/lib/studies/reply/model/email_model.dart
+++ b/lib/studies/reply/model/email_model.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 class Email {
   Email({
     this.id,
diff --git a/lib/studies/reply/model/email_store.dart b/lib/studies/reply/model/email_store.dart
index 8da3c28839..f14a8074d5 100644
--- a/lib/studies/reply/model/email_store.dart
+++ b/lib/studies/reply/model/email_store.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/widgets.dart';
 
 import 'email_model.dart';
diff --git a/lib/studies/reply/profile_avatar.dart b/lib/studies/reply/profile_avatar.dart
index 0e619d550c..3bdbbeedbb 100644
--- a/lib/studies/reply/profile_avatar.dart
+++ b/lib/studies/reply/profile_avatar.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 class ProfileAvatar extends StatelessWidget {
diff --git a/lib/studies/reply/routes.dart b/lib/studies/reply/routes.dart
index 8671abec52..940ee332ae 100644
--- a/lib/studies/reply/routes.dart
+++ b/lib/studies/reply/routes.dart
@@ -2,5 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 const String homeRoute = '/reply';
 const String composeRoute = '/reply/compose';
diff --git a/lib/studies/reply/search_page.dart b/lib/studies/reply/search_page.dart
index b01e068a8d..70bcec2958 100644
--- a/lib/studies/reply/search_page.dart
+++ b/lib/studies/reply/search_page.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/studies/reply/model/email_store.dart';
 import 'package:provider/provider.dart';
diff --git a/lib/studies/reply/waterfall_notched_rectangle.dart b/lib/studies/reply/waterfall_notched_rectangle.dart
index 92597c32b0..6aab072d56 100644
--- a/lib/studies/reply/waterfall_notched_rectangle.dart
+++ b/lib/studies/reply/waterfall_notched_rectangle.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'dart:math' as math;
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/shrine/app.dart b/lib/studies/shrine/app.dart
index 77b9bb3131..1b5a811f8a 100644
--- a/lib/studies/shrine/app.dart
+++ b/lib/studies/shrine/app.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/data/gallery_options.dart';
diff --git a/lib/studies/shrine/backdrop.dart b/lib/studies/shrine/backdrop.dart
index eb0cbfd386..92476e6443 100644
--- a/lib/studies/shrine/backdrop.dart
+++ b/lib/studies/shrine/backdrop.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/shrine/category_menu_page.dart b/lib/studies/shrine/category_menu_page.dart
index b9d5ceb920..24b954310d 100644
--- a/lib/studies/shrine/category_menu_page.dart
+++ b/lib/studies/shrine/category_menu_page.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/data/gallery_options.dart';
diff --git a/lib/studies/shrine/colors.dart b/lib/studies/shrine/colors.dart
index 68731bc413..d8bee6768b 100644
--- a/lib/studies/shrine/colors.dart
+++ b/lib/studies/shrine/colors.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 const Color shrinePink50 = Color(0xFFFEEAE6);
diff --git a/lib/studies/shrine/expanding_bottom_sheet.dart b/lib/studies/shrine/expanding_bottom_sheet.dart
index ad308637e8..4fa275f035 100644
--- a/lib/studies/shrine/expanding_bottom_sheet.dart
+++ b/lib/studies/shrine/expanding_bottom_sheet.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/shrine/home.dart b/lib/studies/shrine/home.dart
index 9ee3b29910..9822fa6f08 100644
--- a/lib/studies/shrine/home.dart
+++ b/lib/studies/shrine/home.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/semantics.dart';
 import 'package:gallery/data/gallery_options.dart';
diff --git a/lib/studies/shrine/login.dart b/lib/studies/shrine/login.dart
index 6ea5aed383..6c685682d7 100644
--- a/lib/studies/shrine/login.dart
+++ b/lib/studies/shrine/login.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/shrine/model/app_state_model.dart b/lib/studies/shrine/model/app_state_model.dart
index f7dd1c78b5..9075a6d5a3 100644
--- a/lib/studies/shrine/model/app_state_model.dart
+++ b/lib/studies/shrine/model/app_state_model.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:gallery/studies/shrine/model/product.dart';
 import 'package:gallery/studies/shrine/model/products_repository.dart';
 import 'package:scoped_model/scoped_model.dart';
diff --git a/lib/studies/shrine/model/product.dart b/lib/studies/shrine/model/product.dart
index 8f2cbf7675..716e5fa7e0 100644
--- a/lib/studies/shrine/model/product.dart
+++ b/lib/studies/shrine/model/product.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/shrine/model/products_repository.dart b/lib/studies/shrine/model/products_repository.dart
index fd33039d4a..2c0e3787ae 100644
--- a/lib/studies/shrine/model/products_repository.dart
+++ b/lib/studies/shrine/model/products_repository.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/studies/shrine/model/product.dart';
 
diff --git a/lib/studies/shrine/page_status.dart b/lib/studies/shrine/page_status.dart
index c3bea6d3b2..50fc516615 100644
--- a/lib/studies/shrine/page_status.dart
+++ b/lib/studies/shrine/page_status.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:gallery/layout/adaptive.dart';
diff --git a/lib/studies/shrine/routes.dart b/lib/studies/shrine/routes.dart
index 5dad847672..430d4ff568 100644
--- a/lib/studies/shrine/routes.dart
+++ b/lib/studies/shrine/routes.dart
@@ -2,5 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 const String loginRoute = '/shrine/login';
 const String homeRoute = '/shrine';
diff --git a/lib/studies/shrine/scrim.dart b/lib/studies/shrine/scrim.dart
index 3a31870225..3e84762b15 100644
--- a/lib/studies/shrine/scrim.dart
+++ b/lib/studies/shrine/scrim.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 class Scrim extends StatelessWidget {
diff --git a/lib/studies/shrine/shopping_cart.dart b/lib/studies/shrine/shopping_cart.dart
index ce32c61340..451b91f9b2 100644
--- a/lib/studies/shrine/shopping_cart.dart
+++ b/lib/studies/shrine/shopping_cart.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/rendering.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/shrine/supplemental/asymmetric_view.dart b/lib/studies/shrine/supplemental/asymmetric_view.dart
index 90d88a7e5c..19af172721 100644
--- a/lib/studies/shrine/supplemental/asymmetric_view.dart
+++ b/lib/studies/shrine/supplemental/asymmetric_view.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/shrine/supplemental/balanced_layout.dart b/lib/studies/shrine/supplemental/balanced_layout.dart
index 408bc5d957..1e465cb292 100644
--- a/lib/studies/shrine/supplemental/balanced_layout.dart
+++ b/lib/studies/shrine/supplemental/balanced_layout.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:gallery/studies/shrine/model/product.dart';
diff --git a/lib/studies/shrine/supplemental/cut_corners_border.dart b/lib/studies/shrine/supplemental/cut_corners_border.dart
index 22baf6d316..57c9d1437f 100644
--- a/lib/studies/shrine/supplemental/cut_corners_border.dart
+++ b/lib/studies/shrine/supplemental/cut_corners_border.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:ui' show lerpDouble;
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/shrine/supplemental/desktop_product_columns.dart b/lib/studies/shrine/supplemental/desktop_product_columns.dart
index 104352b0e3..655bcf6109 100644
--- a/lib/studies/shrine/supplemental/desktop_product_columns.dart
+++ b/lib/studies/shrine/supplemental/desktop_product_columns.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:math';
 
 import 'package:flutter/material.dart';
diff --git a/lib/studies/shrine/supplemental/layout_cache.dart b/lib/studies/shrine/supplemental/layout_cache.dart
index 0c78ff65e3..37222e0d0e 100644
--- a/lib/studies/shrine/supplemental/layout_cache.dart
+++ b/lib/studies/shrine/supplemental/layout_cache.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 class LayoutCache extends InheritedWidget {
diff --git a/lib/studies/shrine/supplemental/product_card.dart b/lib/studies/shrine/supplemental/product_card.dart
index 74eccb04f4..86880f1191 100644
--- a/lib/studies/shrine/supplemental/product_card.dart
+++ b/lib/studies/shrine/supplemental/product_card.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/layout/adaptive.dart';
diff --git a/lib/studies/shrine/supplemental/product_columns.dart b/lib/studies/shrine/supplemental/product_columns.dart
index d8a7a32db0..7536dccedb 100644
--- a/lib/studies/shrine/supplemental/product_columns.dart
+++ b/lib/studies/shrine/supplemental/product_columns.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:gallery/studies/shrine/model/product.dart';
diff --git a/lib/studies/shrine/theme.dart b/lib/studies/shrine/theme.dart
index 8e33bb9176..138dc3ec0a 100644
--- a/lib/studies/shrine/theme.dart
+++ b/lib/studies/shrine/theme.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:gallery/layout/letter_spacing.dart';
diff --git a/lib/studies/shrine/triangle_category_indicator.dart b/lib/studies/shrine/triangle_category_indicator.dart
index ce97eee642..03050b6206 100644
--- a/lib/studies/shrine/triangle_category_indicator.dart
+++ b/lib/studies/shrine/triangle_category_indicator.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:gallery/studies/shrine/colors.dart';
diff --git a/lib/studies/starter/app.dart b/lib/studies/starter/app.dart
index 65eb2527d5..4d05ec4465 100644
--- a/lib/studies/starter/app.dart
+++ b/lib/studies/starter/app.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:gallery/data/gallery_options.dart';
diff --git a/lib/studies/starter/home.dart b/lib/studies/starter/home.dart
index ca68ce8391..07fb62953e 100644
--- a/lib/studies/starter/home.dart
+++ b/lib/studies/starter/home.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
diff --git a/lib/studies/starter/routes.dart b/lib/studies/starter/routes.dart
index 1045889a6b..b8e4db06ca 100644
--- a/lib/studies/starter/routes.dart
+++ b/lib/studies/starter/routes.dart
@@ -2,4 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 const String defaultRoute = '/starter';
diff --git a/lib/themes/gallery_theme_data.dart b/lib/themes/gallery_theme_data.dart
index 6459864fb6..c9602ac9fc 100644
--- a/lib/themes/gallery_theme_data.dart
+++ b/lib/themes/gallery_theme_data.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:google_fonts/google_fonts.dart';
 
diff --git a/lib/themes/material_demo_theme_data.dart b/lib/themes/material_demo_theme_data.dart
index 9df9528cc7..51c0fd9788 100644
--- a/lib/themes/material_demo_theme_data.dart
+++ b/lib/themes/material_demo_theme_data.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 
diff --git a/pubspec.yaml b/pubspec.yaml
index 4ec069030b..329c2db47b 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -5,7 +5,7 @@ version: 2.9.3+020903
 
 environment:
   flutter: ^2.6.0-0 # Kept relatively close to master channel version
-  sdk: ">=2.6.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
   flutter:
diff --git a/test/code_segments_test.dart b/test/code_segments_test.dart
index 0645da4845..bf97861a1e 100644
--- a/test/code_segments_test.dart
+++ b/test/code_segments_test.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:test/test.dart';
 
 import '../tool/codeviewer_cli/segment_generator.dart';
diff --git a/test/demo_descriptions_test.dart b/test/demo_descriptions_test.dart
index b9f4e61694..854dbf1a66 100644
--- a/test/demo_descriptions_test.dart
+++ b/test/demo_descriptions_test.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:collection/collection.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations_en.dart';
 import 'package:flutter_test/flutter_test.dart';
diff --git a/test/l10n_test.dart b/test/l10n_test.dart
index f17a71ab33..66066f607f 100644
--- a/test/l10n_test.dart
+++ b/test/l10n_test.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:test/test.dart';
 
 import '../tool/l10n_cli/l10n_cli.dart';
diff --git a/test/pages/home_test.dart b/test/pages/home_test.dart
index 778f08da65..04b668874e 100644
--- a/test/pages/home_test.dart
+++ b/test/pages/home_test.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_gen/gen_l10n/gallery_localizations.dart';
 import 'package:flutter_test/flutter_test.dart';
diff --git a/test/restoration_test.dart b/test/restoration_test.dart
index 34970c6940..20edcabc5c 100644
--- a/test/restoration_test.dart
+++ b/test/restoration_test.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:gallery/data/demos.dart';
diff --git a/test/utils.dart b/test/utils.dart
index d6c90e4934..80a932f869 100644
--- a/test/utils.dart
+++ b/test/utils.dart
@@ -2,5 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 // Replace Windows line endings with Unix line endings
 String standardizeLineEndings(String str) => str.replaceAll('\r\n', '\n');
diff --git a/test/widget_test.dart b/test/widget_test.dart
index 5a87de6970..4fd9f94d71 100644
--- a/test/widget_test.dart
+++ b/test/widget_test.dart
@@ -2,4 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 void main() {}
diff --git a/test_benchmarks/benchmarks/client.dart b/test_benchmarks/benchmarks/client.dart
index f98193ba68..817d0f0bb2 100644
--- a/test_benchmarks/benchmarks/client.dart
+++ b/test_benchmarks/benchmarks/client.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'package:web_benchmarks/client.dart';
 
 import 'common.dart';
diff --git a/test_benchmarks/benchmarks/common.dart b/test_benchmarks/benchmarks/common.dart
index 67ff142cf8..a30aee26ad 100644
--- a/test_benchmarks/benchmarks/common.dart
+++ b/test_benchmarks/benchmarks/common.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 const String galleryBenchmarkPrefix = 'gallery_v2';
 
 const String galleryStudiesPerf = '${galleryBenchmarkPrefix}_studies_perf';
diff --git a/test_benchmarks/benchmarks/gallery_automator.dart b/test_benchmarks/benchmarks/gallery_automator.dart
index 4f5588114c..6f7a4acc54 100644
--- a/test_benchmarks/benchmarks/gallery_automator.dart
+++ b/test_benchmarks/benchmarks/gallery_automator.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:io';
 
 import 'package:flutter/material.dart';
diff --git a/test_benchmarks/benchmarks/gallery_recorder.dart b/test_benchmarks/benchmarks/gallery_recorder.dart
index f55b5adfb8..78e97c33d8 100644
--- a/test_benchmarks/benchmarks/gallery_recorder.dart
+++ b/test_benchmarks/benchmarks/gallery_recorder.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:web_benchmarks/client.dart';
 
diff --git a/test_benchmarks/benchmarks/project_root_directory.dart b/test_benchmarks/benchmarks/project_root_directory.dart
index ab5e93b38d..17ed9069c0 100644
--- a/test_benchmarks/benchmarks/project_root_directory.dart
+++ b/test_benchmarks/benchmarks/project_root_directory.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'dart:io';
 import 'package:path/path.dart' as path;
 
diff --git a/test_benchmarks/benchmarks/run_benchmarks.dart b/test_benchmarks/benchmarks/run_benchmarks.dart
index 50539b4596..e7bde22a27 100644
--- a/test_benchmarks/benchmarks/run_benchmarks.dart
+++ b/test_benchmarks/benchmarks/run_benchmarks.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'dart:convert' show JsonEncoder;
 import 'dart:io';
 
diff --git a/test_benchmarks/benchmarks/scroll.dart b/test_benchmarks/benchmarks/scroll.dart
index 04eb1dcb07..5caaae2696 100644
--- a/test_benchmarks/benchmarks/scroll.dart
+++ b/test_benchmarks/benchmarks/scroll.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 
 import 'package:flutter/material.dart';
diff --git a/test_benchmarks/benchmarks_test.dart b/test_benchmarks/benchmarks_test.dart
index 3eaed071c7..5aa2fa3f49 100644
--- a/test_benchmarks/benchmarks_test.dart
+++ b/test_benchmarks/benchmarks_test.dart
@@ -1,3 +1,5 @@
+// @dart=2.9
+
 import 'dart:convert' show JsonEncoder;
 import 'dart:io';
 
diff --git a/test_benchmarks/web_bundle_size_test.dart b/test_benchmarks/web_bundle_size_test.dart
index 3cb9771f6c..e6a8fb35e0 100644
--- a/test_benchmarks/web_bundle_size_test.dart
+++ b/test_benchmarks/web_bundle_size_test.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
diff --git a/test_driver/transitions_perf.dart b/test_driver/transitions_perf.dart
index 05c6bbef3e..19841503dd 100644
--- a/test_driver/transitions_perf.dart
+++ b/test_driver/transitions_perf.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:convert' show JsonEncoder;
 
diff --git a/test_driver/transitions_perf_test.dart b/test_driver/transitions_perf_test.dart
index 15a1c9d969..8c0c8c6c29 100644
--- a/test_driver/transitions_perf_test.dart
+++ b/test_driver/transitions_perf_test.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:convert' show json;
 import 'dart:io' show sleep, stdout;
diff --git a/test_goldens/demo_test.dart b/test_goldens/demo_test.dart
index 5ef0a626ff..53e2232164 100644
--- a/test_goldens/demo_test.dart
+++ b/test_goldens/demo_test.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
diff --git a/test_goldens/flutter_test_config.dart b/test_goldens/flutter_test_config.dart
index 36634ff3e9..46fb3c97e4 100644
--- a/test_goldens/flutter_test_config.dart
+++ b/test_goldens/flutter_test_config.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:io';
 
diff --git a/test_goldens/home_test.dart b/test_goldens/home_test.dart
index 470b58ad96..0df37e6275 100644
--- a/test_goldens/home_test.dart
+++ b/test_goldens/home_test.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/cupertino.dart';
 import 'package:flutter_test/flutter_test.dart';
 
diff --git a/test_goldens/testing/font_loader.dart b/test_goldens/testing/font_loader.dart
index d79772f8d0..1293223667 100644
--- a/test_goldens/testing/font_loader.dart
+++ b/test_goldens/testing/font_loader.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:convert';
 import 'dart:io';
 
diff --git a/test_goldens/testing/precache_images.dart b/test_goldens/testing/precache_images.dart
index e8f2924fd0..58a1feffbc 100644
--- a/test_goldens/testing/precache_images.dart
+++ b/test_goldens/testing/precache_images.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
diff --git a/test_goldens/testing/util.dart b/test_goldens/testing/util.dart
index 0704b92736..f0dc61aa80 100644
--- a/test_goldens/testing/util.dart
+++ b/test_goldens/testing/util.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
 
diff --git a/tool/codeviewer_cli/main.dart b/tool/codeviewer_cli/main.dart
index ff624bb2c9..798457fe39 100644
--- a/tool/codeviewer_cli/main.dart
+++ b/tool/codeviewer_cli/main.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:io';
 
 import 'package:args/args.dart';
diff --git a/tool/codeviewer_cli/prehighlighter.dart b/tool/codeviewer_cli/prehighlighter.dart
index 6038c71cfd..aa3b2a93f4 100644
--- a/tool/codeviewer_cli/prehighlighter.dart
+++ b/tool/codeviewer_cli/prehighlighter.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:string_scanner/string_scanner.dart';
 
 abstract class SyntaxPrehighlighter {
diff --git a/tool/codeviewer_cli/segment_generator.dart b/tool/codeviewer_cli/segment_generator.dart
index 004e246c5f..541f013874 100644
--- a/tool/codeviewer_cli/segment_generator.dart
+++ b/tool/codeviewer_cli/segment_generator.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
@@ -11,6 +13,7 @@ import 'prehighlighter.dart';
 const codeSegmentsSourcePath = 'lib/demos';
 const codeSegmentsPath = 'lib/codeviewer/code_segments.dart';
 
+// TODO(amanv8060): Remove dart version once code_style is migrated to null safety
 const _globalPrologue =
     '''// This file is automatically generated by codeviewer_cli.
 // Do not edit this file.
@@ -19,6 +22,8 @@ const _globalPrologue =
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'package:flutter/material.dart';
 import 'package:gallery/codeviewer/code_style.dart';
 
diff --git a/tool/grind.dart b/tool/grind.dart
index 8db3503c9a..7df19e669a 100644
--- a/tool/grind.dart
+++ b/tool/grind.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:io';
 
diff --git a/tool/l10n_cli/l10n_cli.dart b/tool/l10n_cli/l10n_cli.dart
index 3767eb0ca2..52b231a6c3 100644
--- a/tool/l10n_cli/l10n_cli.dart
+++ b/tool/l10n_cli/l10n_cli.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
diff --git a/tool/l10n_cli/main.dart b/tool/l10n_cli/main.dart
index 61bdc2cb76..b5ae05fb94 100644
--- a/tool/l10n_cli/main.dart
+++ b/tool/l10n_cli/main.dart
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart=2.9
+
 import 'l10n_cli.dart';
 
 void main(List<String> arguments) {