Invalidate the WebStaticAssets target if the web sdk changes. (#123739)

Invalidate the `WebStaticAssets` target if the web sdk changes.

Author: eyebrowsoffire

packages/flutter_tools/lib/src/build_system/targets/web.dart

@@ -494,8 +494,8 @@ class WebReleaseBundle extends Target {
 /// Static assets provided by the Flutter SDK that do not change, such as
 /// CanvasKit.
 ///
-/// These assets can be cached forever and are only invalidated when the
-/// Flutter SDK is upgraded to a new version.
+/// These assets can be cached until a new version of the flutter web sdk is
+/// downloaded.
 class WebBuiltInAssets extends Target {
   const WebBuiltInAssets(this.fileSystem, {required this.isWasm});
 
@@ -512,7 +512,9 @@ class WebBuiltInAssets extends Target {
   List<String> get depfiles => const <String>[];
 
   @override
-  List<Source> get inputs => const <Source>[];
+  List<Source> get inputs => const <Source>[
+    Source.hostArtifact(HostArtifact.flutterWebSdk),
+  ];
 
   @override
   List<Source> get outputs => const <Source>[];

packages/flutter_tools/test/general.shard/build_system/targets/web_test.dart

@@ -946,4 +946,26 @@ void main() {
       .childFile('canvaskit.wasm')
       .existsSync(), true);
   }));
+
+  test('wasm copies over canvaskit again if the web sdk changes', () => testbed.run(() async {
+    final File canvasKitInput = globals.fs.file('bin/cache/flutter_web_sdk/canvaskit/canvaskit.wasm')
+      ..createSync(recursive: true);
+    canvasKitInput.writeAsStringSync('foo', flush: true);
+
+    await WebBuiltInAssets(globals.fs, isWasm: true).build(environment);
+
+    final File canvasKitOutputBefore = environment.outputDir.childDirectory('canvaskit')
+      .childFile('canvaskit.wasm');
+    expect(canvasKitOutputBefore.existsSync(), true);
+    expect(canvasKitOutputBefore.readAsStringSync(), 'foo');
+
+    canvasKitInput.writeAsStringSync('bar', flush: true);
+
+    await WebBuiltInAssets(globals.fs, isWasm: true).build(environment);
+
+    final File canvasKitOutputAfter = environment.outputDir.childDirectory('canvaskit')
+      .childFile('canvaskit.wasm');
+    expect(canvasKitOutputAfter.existsSync(), true);
+    expect(canvasKitOutputAfter.readAsStringSync(), 'bar');
+  }));
 }