Massage the JS interop around didCreateEngineInitializer (#38147)

* Massage the JS interop around `didCreateEngineInitializer` to make it work
better with dart2wasm.

* Make the whole type hierarchy more explicit.

* Address Joshua's comments.

Author: eyebrowsoffire

lib/web_ui/lib/initialization.dart

@@ -79,19 +79,15 @@ Future<void> webOnlyWarmupEngine({
     },
   );
 
-  // Should the app "autoStart"?
-  bool autoStart = true;
-  if (engine.flutter != null && engine.loader != null) {
-    autoStart = engine.didCreateEngineInitializer == null;
-  }
-  if (autoStart) {
+  final engine.FlutterLoader? loader = engine.flutter?.loader;
+  if (loader == null || loader.isAutoStart) {
     // The user does not want control of the app, bootstrap immediately.
     engine.domWindow.console.debug('Flutter Web Bootstrap: Auto.');
     await bootstrap.autoStart();
   } else {
     // Yield control of the bootstrap procedure to the user.
     engine.domWindow.console.debug('Flutter Web Bootstrap: Programmatic.');
-    engine.didCreateEngineInitializer!(bootstrap.prepareEngineInitializer());
+    loader.didCreateEngineInitializer(bootstrap.prepareEngineInitializer());
   }
 }
 

lib/web_ui/lib/src/engine/js_interop/js_loader.dart

@@ -6,25 +6,32 @@
 library js_loader;
 
 import 'package:js/js.dart';
+import 'package:js/js_util.dart' as js_util;
 
 import '../configuration.dart';
 import 'js_promise.dart';
 
-/// Typedef for the function that notifies JS that the main entrypoint is up and running.
-/// As a parameter, a [FlutterEngineInitializer] instance is passed to JS, so the
-/// programmer can control the initialization sequence.
-typedef DidCreateEngineInitializerFn = void Function(FlutterEngineInitializer);
+@JS()
+@staticInterop
+class FlutterJS {}
+
+extension FlutterJSExtension on FlutterJS {
+  external FlutterLoader? get loader;
+}
 
 // Both `flutter`, `loader`(_flutter.loader), must be checked for null before
 // `didCreateEngineInitializer` can be safely accessed.
 @JS('_flutter')
-external Object? get flutter;
+external FlutterJS? get flutter;
 
-@JS('_flutter.loader')
-external Object? get loader;
+@JS()
+@staticInterop
+class FlutterLoader {}
 
-@JS('_flutter.loader.didCreateEngineInitializer')
-external DidCreateEngineInitializerFn? get didCreateEngineInitializer;
+extension FlutterLoaderExtension on FlutterLoader {
+  external void didCreateEngineInitializer(FlutterEngineInitializer initializer);
+  bool get isAutoStart => !js_util.hasProperty(this, 'didCreateEngineInitializer');
+}
 
 // FlutterEngineInitializer