[tools] Make SnapshotType.platform non-nullable (#146958)

When performing artifact lookups for `Artifact.genSnapshot` for macOS desktop builds, a `TargetPlatform` is used to determine the name of the tool, typically `gen_snapshot_$TARGET_ARCH`. Formerly, this tool was always named `gen_snapshot`.

The astute reader may ask "but Chris, didn't we support TWO target architectures on iOS and therefore need TWO `gen_snapshot` binaries?" Yes, we did support both armv7 and arm64 target architectures on iOS. But no, we didn't initially have two `gen_snapshot` binaries. We did *build* two `gen_snapshots`:
   * A 32-bit x86 binary that emitted armv7 AOT code
   * A 64-bit x64 binary that emitted arm64 AOT code 

At the time, the bitness of the `gen_snapshot` tool needed to match the bitness of the target architecture, and to avoid having to do a lot of work plumbing through suffixed `gen_snapshot` names, the author of that work (who, as evidenced by this patch, is still paying for his code crimes) elected to "cleverly" lipo the two together into a single multi-architecture macOS binary still named `gen_snapshot`. See: flutter/engine#4948

This was later remediated over the course of several patches, including:
   * flutter/engine#10430
   * flutter/engine#22818
   * flutter/flutter#37445 

However, there were still cases (notably `--local-engine` workflows in the tool) where we weren't computing the target platform and thus referenced the generic `gen_snapshot` tool.
See: flutter/flutter#38933
Fixed in: flutter/engine#28345

The test removed in this PR, which ensured that null `SnapshotType.platform` was supported was introduced in flutter/flutter#11924 as a followup to flutter/flutter#11820 when the snapshotting logic was originally extracted to the `GenSnapshot` class, and most invocations still passed a null target platform.

Since there are no longer any cases where `TargetPlatform` isn't passed when looking up `Artifact.genSnapshot`, we can safely make the platform non-nullable and remove the test.

This is pre-factoring towards the removal of the generic `gen_snapshot` artifact from the macOS host binaries (which are currently unused since we never pass a null `TargetPlatform`), which is pre-factoring towards the goal of building `gen_snapshot` binaries with an arm64 host architecture, and eliminate the need to use Rosetta during iOS and macOS Flutter builds.

Part of: flutter/flutter#101138
Umbrella issue: flutter/flutter#103386
Umbrella issue: flutter/flutter#69157

No new tests since the behaviour is enforced by the compiler.

packages/flutter_tools/lib/src/artifacts.dart

@@ -578,7 +578,7 @@ class CachedArtifacts implements Artifacts {
       case TargetPlatform.linux_arm64:
       case TargetPlatform.windows_x64:
       case TargetPlatform.windows_arm64:
-        return _getDesktopArtifactPath(artifact, platform, mode);
+        return _getDesktopArtifactPath(artifact, platform!, mode);
       case TargetPlatform.fuchsia_arm64:
       case TargetPlatform.fuchsia_x64:
         return _getFuchsiaArtifactPath(artifact, platform!, mode!);
@@ -594,18 +594,18 @@ class CachedArtifacts implements Artifacts {
     return _fileSystem.path.basename(_getEngineArtifactsPath(platform, mode)!);
   }
 
-  String _getDesktopArtifactPath(Artifact artifact, TargetPlatform? platform, BuildMode? mode) {
+  String _getDesktopArtifactPath(Artifact artifact, TargetPlatform platform, BuildMode? mode) {
     // When platform is null, a generic host platform artifact is being requested
     // and not the gen_snapshot for darwin as a target platform.
-    if (platform != null && artifact == Artifact.genSnapshot) {
+    if (artifact == Artifact.genSnapshot) {
       final String engineDir = _getEngineArtifactsPath(platform, mode)!;
       return _fileSystem.path.join(engineDir, _artifactToFileName(artifact, _platform));
     }
-    if (platform != null && artifact == Artifact.flutterMacOSFramework) {
+    if (artifact == Artifact.flutterMacOSFramework) {
       final String engineDir = _getEngineArtifactsPath(platform, mode)!;
       return _getMacOSEngineArtifactPath(engineDir, _fileSystem, _platform);
     }
-    return _getHostArtifactPath(artifact, platform ?? _currentHostPlatform(_platform, _operatingSystemUtils), mode);
+    return _getHostArtifactPath(artifact, platform, mode);
   }
 
   String _getAndroidArtifactPath(Artifact artifact, TargetPlatform platform, BuildMode mode) {

packages/flutter_tools/lib/src/base/build.dart

@@ -16,7 +16,7 @@ import 'process.dart';
 class SnapshotType {
   SnapshotType(this.platform, this.mode);
 
-  final TargetPlatform? platform;
+  final TargetPlatform platform;
   final BuildMode mode;
 
   @override

packages/flutter_tools/test/general.shard/base/build_test.dart

@@ -49,12 +49,6 @@ const List<String> kDefaultClang = <String>[
 ];
 
 void main() {
-  group('SnapshotType', () {
-    test('does not throw, if target platform is null', () {
-      expect(() => SnapshotType(null, BuildMode.release), returnsNormally);
-    });
-  });
-
   group('GenSnapshot', () {
     late GenSnapshot genSnapshot;
     late Artifacts artifacts;