[pigeon] Fixed decoding code for null-safe classes.

packages/pigeon/lib/dart_generator.dart

@@ -178,11 +178,11 @@ void _writeFlutterApi(
 String _addGenericTypes(String dataType, String nullTag) {
   switch (dataType) {
     case 'List':
-      return 'List<Object$nullTag>';
+      return 'List<Object$nullTag>$nullTag';
     case 'Map':
-      return 'Map<Object$nullTag, Object$nullTag>';
+      return 'Map<Object$nullTag, Object$nullTag>$nullTag';
     default:
-      return dataType;
+      return '$dataType$nullTag';
   }
 }
 
@@ -212,7 +212,7 @@ void generateDart(DartOptions opt, Root root, StringSink sink) {
     indent.scoped('{', '}', () {
       for (Field field in klass.fields) {
         final String datatype =
-            '${_addGenericTypes(field.dataType, nullTag)}$nullTag';
+            '${_addGenericTypes(field.dataType, nullTag)}';
         indent.writeln('$datatype ${field.name};');
       }
       if (klass.fields.isNotEmpty) {

packages/pigeon/platform_tests/flutter_null_safe_unit_tests/.gitignore

@@ -0,0 +1,46 @@
+# Miscellaneous
+*.class
+*.log
+*.pyc
+*.swp
+.DS_Store
+.atom/
+.buildlog/
+.history
+.svn/
+
+# IntelliJ related
+*.iml
+*.ipr
+*.iws
+.idea/
+
+# The .vscode folder contains launch configuration and tasks you configure in
+# VS Code which you may wish to be included in version control, so this line
+# is commented out by default.
+#.vscode/
+
+# Flutter/Dart/Pub related
+**/doc/api/
+**/ios/Flutter/.last_build_id
+.dart_tool/
+.flutter-plugins
+.flutter-plugins-dependencies
+.packages
+.pub-cache/
+.pub/
+/build/
+
+# Web related
+lib/generated_plugin_registrant.dart
+
+# Symbolication related
+app.*.symbols
+
+# Obfuscation related
+app.*.map.json
+
+# Android Studio will place build artifacts here
+/android/app/debug
+/android/app/profile
+/android/app/release

packages/pigeon/platform_tests/flutter_null_safe_unit_tests/.metadata

@@ -0,0 +1,10 @@
+# This file tracks properties of this Flutter project.
+# Used by Flutter tool to assess capabilities and perform upgrades etc.
+#
+# This file should be version controlled and should not be manually edited.
+
+version:
+  revision: 6db3d61ed4a58ba89140d7fe1fd294b598cc29c5
+  channel: master
+
+project_type: app

packages/pigeon/platform_tests/flutter_null_safe_unit_tests/README.md

@@ -0,0 +1,3 @@
+# flutter_unit_tests
+
+Unit test scaffold for null safe Flutter projects.

packages/pigeon/platform_tests/flutter_null_safe_unit_tests/lib/.gitignore

@@ -0,0 +1 @@
+null_safe_pigeon.dart

packages/pigeon/platform_tests/flutter_null_safe_unit_tests/pubspec.yaml

@@ -0,0 +1,17 @@
+name: flutter_unit_tests
+description: Unit test scaffold for null safe Flutter projects.
+publish_to: 'none' # Remove this line if you wish to publish to pub.dev
+version: 1.0.0+1
+environment:
+  sdk: ">=2.12.0 <3.0.0"
+
+dependencies:
+  flutter:
+    sdk: flutter
+
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+
+flutter:
+  uses-material-design: true

packages/pigeon/platform_tests/flutter_null_safe_unit_tests/test/null_safe_test.dart

@@ -0,0 +1,28 @@
+// Copyright 2020 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_unit_tests/null_safe_pigeon.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('with values filled', () {
+    final SearchReply reply = SearchReply()
+      ..result = 'foo'
+      ..error = 'bar';
+    final Object encoded = reply.encode();
+    final SearchReply decoded = SearchReply.decode(encoded);
+    expect(reply.result, decoded.result);
+    expect(reply.error, decoded.error);
+  });
+
+  test('with null value', () {
+    final SearchReply reply = SearchReply()
+      ..result = 'foo'
+      ..error = null;
+    final Object encoded = reply.encode();
+    final SearchReply decoded = SearchReply.decode(encoded);
+    expect(reply.result, decoded.result);
+    expect(reply.error, decoded.error);
+  });
+}

packages/pigeon/run_tests.sh

@@ -110,6 +110,19 @@ pub run test test/
 ###############################################################################
 pub run pigeon 1> /dev/null
 
+###############################################################################
+# Run unit tests on generated Dart code.
+###############################################################################
+pushd $PWD
+pub run pigeon \
+    --input pigeons/message.dart \
+    --dart_null_safety \
+    --dart_out platform_tests/flutter_null_safe_unit_tests/lib/null_safe_pigeon.dart
+cd platform_tests/flutter_null_safe_unit_tests
+flutter pub get
+flutter test test/null_safe_test.dart 
+popd
+
 ###############################################################################
 # Mock handler flutter tests.
 ###############################################################################