[web] make new image decoder opt-in via experimental flag

lib/web_ui/dev/steps/compile_tests_step.dart

@@ -258,6 +258,14 @@ Future<bool> compileUnitTest(FilePath input, { required bool forCanvasKit }) asy
     '-DFLUTTER_WEB_AUTO_DETECT=false',
     '-DFLUTTER_WEB_USE_SKIA=$forCanvasKit',
 
+    // Enable the image decoder experiment in tests so we can test the new
+    // functionality. WASM decoders are still tested by forcing the value of
+    // `browserSupportsImageDecoder` to false in the test. See also:
+    //
+    // lib/web_ui/test/canvaskit/image_golden_test.dart
+    // TODO(yjbanov): https://github.com/flutter/flutter/issues/95277
+    '-DEXPERIMENTAL_IMAGE_DECODER=true',
+
     '-O2',
     '-o',
     targetFileName, // target path.

lib/web_ui/lib/src/engine/safe_browser_api.dart

@@ -218,9 +218,18 @@ html.CanvasElement? tryCreateCanvasElement(int width, int height) {
 @JS('window.ImageDecoder')
 external Object? get _imageDecoderConstructor;
 
+/// Hides `image_web_codecs.dart` behind a flag.
+// TODO(yjbanov): https://github.com/flutter/flutter/issues/95277
+const bool _imageDecoderExperimentEnabled = bool.fromEnvironment(
+  'EXPERIMENTAL_IMAGE_DECODER',
+  defaultValue: false,
+);
+
 /// Whether the current browser supports `ImageDecoder`.
 bool browserSupportsImageDecoder =
-    _imageDecoderConstructor != null && browserEngine == BrowserEngine.blink;
+  _imageDecoderExperimentEnabled &&
+  _imageDecoderConstructor != null &&
+  browserEngine == BrowserEngine.blink;
 
 /// Sets the value of [browserSupportsImageDecoder] to its default value.
 void debugResetBrowserSupportsImageDecoder() {