[web] make addView/removeView functions sync

lib/web_ui/lib/src/engine/app_bootstrap.dart

@@ -68,11 +68,11 @@ class AppBootstrap {
   /// Represents the App that was just started, and its JS API.
   FlutterApp _prepareFlutterApp() {
     return FlutterApp(
-      addView: (JsFlutterViewOptions options) async {
+      addView: (JsFlutterViewOptions options) {
         assert(configuration.multiViewEnabled, 'Cannot addView when multiView is not enabled');
         return viewManager.createAndRegisterView(options).viewId;
       },
-      removeView: (int viewId) async {
+      removeView: (int viewId) {
         assert(configuration.multiViewEnabled, 'Cannot removeView when multiView is not enabled');
         return viewManager.disposeAndUnregisterView(viewId);
       }

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

@@ -67,10 +67,8 @@ abstract class FlutterApp {
     required RemoveFlutterViewFn removeView,
   }) =>
       FlutterApp._(
-        addView: ((JsFlutterViewOptions options) =>
-            futureToPromise(addView(options) as Future<JSAny>)).toJS,
-        removeView: ((int id) =>
-            futureToPromise(removeView(id) as Future<JSObject?>)).toJS,
+        addView: addView.toJS,
+        removeView: ((JSNumber id) => removeView(id.toDartInt)).toJS,
       );
   external factory FlutterApp._({
     required JSFunction addView,
@@ -81,9 +79,9 @@ abstract class FlutterApp {
 /// Typedef for the function that adds a new view to the app.
 ///
 /// Returns the ID of the newly created view.
-typedef AddFlutterViewFn = Future<int> Function(JsFlutterViewOptions);
+typedef AddFlutterViewFn = int Function(JsFlutterViewOptions);
 
 /// Typedef for the function that removes a view from the app.
 ///
 /// Returns the configuration used to create the view.
-typedef RemoveFlutterViewFn = Future<JsFlutterViewOptions?> Function(int);
+typedef RemoveFlutterViewFn = JsFlutterViewOptions? Function(int);

lib/web_ui/test/engine/app_bootstrap_test.dart

@@ -28,6 +28,8 @@ void testMain() {
   });
 
   Future<void> mockInit ([JsFlutterConfiguration? configuration]) async {
+    debugOverrideJsConfiguration(configuration);
+    addTearDown(() => debugOverrideJsConfiguration(null));
     initCalled = callOrder++;
     await Future<void>.delayed(const Duration(milliseconds: 1));
   }
@@ -128,5 +130,41 @@ void testMain() {
       expect(getJsProperty<dynamic>(maybeApp, 'addView'), isA<Function>());
       expect(getJsProperty<dynamic>(maybeApp, 'removeView'), isA<Function>());
     });
+    test('addView/removeView respectively adds/removes view', () async {
+      final AppBootstrap bootstrap = AppBootstrap(
+        initializeEngine: mockInit,
+        runApp: mockRunApp,
+      );
+
+      final FlutterEngineInitializer engineInitializer = bootstrap.prepareEngineInitializer();
+
+      final Object appInitializer = await promiseToFuture<Object>(callMethod<Object>(
+        engineInitializer,
+        'initializeEngine',
+        <dynamic>[jsify(<String, Object?>{
+          'multiViewEnabled': true,
+        })]
+      ));
+      final Object maybeApp = await promiseToFuture<Object>(callMethod<Object>(
+        appInitializer,
+        'runApp',
+        <Object?>[]
+      ));
+      final int viewId = callMethod<num>(
+        maybeApp,
+        'addView',
+        <dynamic>[jsify(<String, Object?>{
+          'hostElement': createDomElement('div'),
+        })]
+      ).toInt();
+      expect(bootstrap.viewManager[viewId], isNotNull);
+
+      callMethod<Object>(
+        maybeApp,
+        'removeView',
+        <Object>[viewId]
+      );
+      expect(bootstrap.viewManager[viewId], isNull);
+    });
   });
 }