[web] Upstream some fixes for JS interop on dart2wasm.

lib/web_ui/lib/initialization.dart

@@ -81,11 +81,8 @@ Future<void> webOnlyWarmupEngine({
 
   // Should the app "autoStart"?
   bool autoStart = true;
-  try {
+  if (engine.flutter != null && engine.loader != null) {
     autoStart = engine.didCreateEngineInitializer == null;
-  } catch (e) {
-    // Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'loader')
-    autoStart = true;
   }
   if (autoStart) {
     // The user does not want control of the app, bootstrap immediately.

lib/web_ui/lib/src/engine/dom.dart

@@ -1276,8 +1276,7 @@ class DomStyleSheet {}
 class DomCSSStyleSheet extends DomStyleSheet {}
 
 extension DomCSSStyleSheetExtension on DomCSSStyleSheet {
-  List<DomCSSRule> get cssRules =>
-      js_util.getProperty<List<Object?>>(this, 'cssRules').cast<DomCSSRule>();
+  external DomCSSRuleList get cssRules;
   int insertRule(String rule, [int? index]) => js_util
       .callMethod(this, 'insertRule', <Object>[rule, if (index != null) index]);
 }
@@ -1381,6 +1380,14 @@ extension DomMessageChannelExtension on DomMessageChannel {
   external DomMessagePort get port2;
 }
 
+@JS()
+@staticInterop
+class DomCSSRuleList {}
+
+extension DomCSSRuleListExtension on DomCSSRuleList {
+  external int get length;
+}
+
 DomMessageChannel createDomMessageChannel() =>
     domCallConstructorString('MessageChannel', <Object>[])!
         as DomMessageChannel;

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

@@ -14,7 +14,13 @@ import 'js_promise.dart';
 /// programmer can control the initialization sequence.
 typedef DidCreateEngineInitializerFn = void Function(FlutterEngineInitializer);
 
-// A JS-interop representation of `_flutter?.loader?.didCreateEngineInitializer?`:
+// Both `flutter`, `loader`(_flutter.loader), must be checked for null before
+// `didCreateEngineInitializer` can be safely accessed.
+@JS('_flutter')
+external Object? get flutter;
+
+@JS('_flutter.loader')
+external Object? get loader;
 
 @JS('_flutter.loader.didCreateEngineInitializer')
 external DidCreateEngineInitializerFn? get didCreateEngineInitializer;