Revert mixin changes

lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -69,7 +69,7 @@ class HighContrastSupport {
 ///
 /// This is the central entry point for platform messages and configuration
 /// events from the platform.
-class EnginePlatformDispatcher extends ui.PlatformDispatcher with ViewFocusBinding {
+class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   /// Private constructor, since only dart:ui is supposed to create one of
   /// these.
   EnginePlatformDispatcher() {
@@ -79,7 +79,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher with ViewFocusBindi
     _addLocaleChangedListener();
     registerHotRestartListener(dispose);
     AppLifecycleState.instance.addListener(_setAppLifecycleState);
-    initViewFocusBindings();
+    _viewFocusBinding.init();
     domDocument.body?.append(accessibilityPlaceholder);
     _onViewDisposedListener = viewManager.onViewDisposed.listen((_) {
       // Send a metrics changed event to the framework when a view is disposed.
@@ -123,7 +123,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher with ViewFocusBindi
     _removeLocaleChangedListener();
     HighContrastSupport.instance.removeListener(_updateHighContrast);
     AppLifecycleState.instance.removeListener(_setAppLifecycleState);
-    disposeViewFocusBindings();
+    _viewFocusBinding.dispose();
     accessibilityPlaceholder.remove();
     _onViewDisposedListener.cancel();
     viewManager.dispose();
@@ -153,7 +153,6 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher with ViewFocusBindi
     EngineFlutterDisplay.instance,
   ];
 
-  @override
   late final FlutterViewManager viewManager = FlutterViewManager(this);
 
   /// The current list of windows.
@@ -229,6 +228,9 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher with ViewFocusBindi
     }
   }
 
+  late final ViewFocusBinding _viewFocusBinding =
+    ViewFocusBinding(viewManager, invokeOnViewFocusChange);
+
   @override
   ui.ViewFocusChangeCallback? get onViewFocusChange => _onViewFocusChange;
   ui.ViewFocusChangeCallback? _onViewFocusChange;
@@ -241,7 +243,6 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher with ViewFocusBindi
 
   // Engine code should use this method instead of the callback directly.
   // Otherwise zones won't work properly.
-  @override
   void invokeOnViewFocusChange(ui.ViewFocusEvent viewFocusEvent) {
     invoke1<ui.ViewFocusEvent>(
       _onViewFocusChange,

lib/web_ui/lib/src/engine/platform_dispatcher/view_focus_binding.dart

@@ -7,24 +7,26 @@ import 'package:ui/src/engine.dart';
 import 'package:ui/ui.dart' as ui;
 
 /// Tracks the [FlutterView]s focus changes.
-mixin ViewFocusBinding {
-  FlutterViewManager get viewManager;
-  void invokeOnViewFocusChange(ui.ViewFocusEvent viewFocusEvent);
+final class ViewFocusBinding {
+  ViewFocusBinding(this._viewManager, this._onViewFocusChange);
+
+  final FlutterViewManager _viewManager;
+  final ui.ViewFocusChangeCallback _onViewFocusChange;
 
   int? _lastViewId;
   ui.ViewFocusDirection _viewFocusDirection = ui.ViewFocusDirection.forward;
 
   StreamSubscription<int>? _onViewCreatedListener;
 
-  void initViewFocusBindings() {
+  void init() {
     domDocument.body?.addEventListener(_keyDown, _handleKeyDown);
     domDocument.body?.addEventListener(_keyUp, _handleKeyUp);
     domDocument.body?.addEventListener(_focusin, _handleFocusin);
     domDocument.body?.addEventListener(_focusout, _handleFocusout);
-    _onViewCreatedListener = viewManager.onViewCreated.listen(_handleViewCreated);
+    _onViewCreatedListener = _viewManager.onViewCreated.listen(_handleViewCreated);
   }
 
-  void disposeViewFocusBindings() {
+  void dispose() {
     domDocument.body?.removeEventListener(_keyDown, _handleKeyDown);
     domDocument.body?.removeEventListener(_keyUp, _handleKeyUp);
     domDocument.body?.removeEventListener(_focusin, _handleFocusin);
@@ -76,15 +78,15 @@ mixin ViewFocusBinding {
     _markViewAsFocusable(_lastViewId, reachableByKeyboard: true);
     _markViewAsFocusable(viewId, reachableByKeyboard: false);
     _lastViewId = viewId;
-    invokeOnViewFocusChange(event);
+    _onViewFocusChange(event);
   }
 
   int? _viewId(DomElement? element) {
     final DomElement? rootElement = element?.closest(DomManager.flutterViewTagName);
     if (rootElement == null) {
       return null;
     }
-    return viewManager.viewIdForRootElement(rootElement);
+    return _viewManager.viewIdForRootElement(rootElement);
   }
 
   void _handleViewCreated(int viewId) {
@@ -105,7 +107,7 @@ mixin ViewFocusBinding {
     // the flutter view and having it with a zero tabindex messes the focus
     // traversal order when pressing tab or shift tab.
     final int tabIndex = reachableByKeyboard ? 0 : -1;
-    viewManager[viewId]?.dom.rootElement.setAttribute('tabindex', tabIndex);
+    _viewManager[viewId]?.dom.rootElement.setAttribute('tabindex', tabIndex);
   }
 
   static const String _focusin = 'focusin';