Implement PlatformDispatcher.requestViewFocusChange on web. (flutter/engine#50535)

Implement PlatformDispatcher.requestViewFocusChange on web. 

Relevant Issues are:

* Design doc: https://flutter.dev/go/focus-management 
* Focus in web multiview: https://github.com/flutter/flutter/issues/137443
* Platform dispatcher changes: https://github.com/flutter/engine/pull/49841

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

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

@@ -257,7 +257,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     required ui.ViewFocusState state,
     required ui.ViewFocusDirection direction,
   }) {
-    // TODO(tugorez): implement this method. At the moment will be a no op call.
+    _viewFocusBinding.changeViewFocus(viewId, state);
   }
 
   /// A set of views which have rendered in the current `onBeginFrame` or

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

@@ -34,6 +34,19 @@ final class ViewFocusBinding {
     _onViewCreatedListener?.cancel();
   }
 
+  void changeViewFocus(int viewId, ui.ViewFocusState state) {
+    final DomElement? viewElement = _viewManager[viewId]?.dom.rootElement;
+
+    if (state == ui.ViewFocusState.focused) {
+      // Only move the focus to the flutter view if nothing inside it is focused already.
+      if (viewId != _viewId(domDocument.activeElement)) {
+        viewElement?.focus();
+      }
+    } else {
+      viewElement?.blur();
+    }
+  }
+
   late final DomEventListener _handleFocusin = createDomEventListener((DomEvent event) {
     event as DomFocusEvent;
     _handleFocusChange(event.target as DomElement?);

engine/src/flutter/lib/web_ui/test/engine/platform_dispatcher/view_focus_binding_test.dart

@@ -164,6 +164,108 @@ void testMain() {
       expect(dispatchedViewFocusEvents[2].state, ui.ViewFocusState.unfocused);
       expect(dispatchedViewFocusEvents[2].direction, ui.ViewFocusDirection.undefined);
     });
+
+    test('requestViewFocusChange focuses the view', () {
+      final EngineFlutterView view = createAndRegisterView(dispatcher);
+
+      dispatcher.requestViewFocusChange(
+        viewId: view.viewId,
+        state: ui.ViewFocusState.focused,
+        direction: ui.ViewFocusDirection.forward,
+      );
+
+      expect(domDocument.activeElement, view.dom.rootElement);
+
+      expect(dispatchedViewFocusEvents, hasLength(1));
+
+      expect(dispatchedViewFocusEvents[0].viewId, view.viewId);
+      expect(dispatchedViewFocusEvents[0].state, ui.ViewFocusState.focused);
+      expect(dispatchedViewFocusEvents[0].direction, ui.ViewFocusDirection.forward);
+    });
+
+    test('requestViewFocusChange blurs the view', () {
+      final EngineFlutterView view = createAndRegisterView(dispatcher);
+
+      dispatcher.requestViewFocusChange(
+        viewId: view.viewId,
+        state: ui.ViewFocusState.focused,
+        direction: ui.ViewFocusDirection.forward,
+      );
+
+      dispatcher.requestViewFocusChange(
+        viewId: view.viewId,
+        state: ui.ViewFocusState.unfocused,
+        direction: ui.ViewFocusDirection.undefined,
+      );
+
+      expect(domDocument.activeElement, isNot(view.dom.rootElement));
+
+      expect(dispatchedViewFocusEvents, hasLength(2));
+
+      expect(dispatchedViewFocusEvents[0].viewId, view.viewId);
+      expect(dispatchedViewFocusEvents[0].state, ui.ViewFocusState.focused);
+      expect(dispatchedViewFocusEvents[0].direction, ui.ViewFocusDirection.forward);
+
+      expect(dispatchedViewFocusEvents[1].viewId, view.viewId);
+      expect(dispatchedViewFocusEvents[1].state, ui.ViewFocusState.unfocused);
+      expect(dispatchedViewFocusEvents[1].direction, ui.ViewFocusDirection.undefined);
+    });
+
+    test('requestViewFocusChange does nothing if the view does not exist', () {
+      final EngineFlutterView view = createAndRegisterView(dispatcher);
+
+      dispatcher.requestViewFocusChange(
+        viewId: 5094555,
+        state: ui.ViewFocusState.focused,
+        direction: ui.ViewFocusDirection.forward,
+      );
+
+      expect(domDocument.activeElement, isNot(view.dom.rootElement));
+      expect(dispatchedViewFocusEvents, isEmpty);
+    });
+
+    test('requestViewFocusChange does nothing if the view is already focused', () {
+      final EngineFlutterView view = createAndRegisterView(dispatcher);
+
+      dispatcher.requestViewFocusChange(
+        viewId: view.viewId,
+        state: ui.ViewFocusState.focused,
+        direction: ui.ViewFocusDirection.forward,
+      );
+      dispatcher.requestViewFocusChange(
+        viewId: view.viewId,
+        state: ui.ViewFocusState.focused,
+        direction: ui.ViewFocusDirection.forward,
+      );
+
+      expect(dispatchedViewFocusEvents, hasLength(1));
+
+      expect(dispatchedViewFocusEvents[0].viewId, view.viewId);
+      expect(dispatchedViewFocusEvents[0].state, ui.ViewFocusState.focused);
+      expect(dispatchedViewFocusEvents[0].direction, ui.ViewFocusDirection.forward);
+    });
+
+    test('requestViewFocusChange does not move the focus to the view', () {
+      final DomElement input = createDomElement('input');
+      final EngineFlutterView view = createAndRegisterView(dispatcher);
+
+      view.dom.rootElement.append(input);
+      input.focus();
+
+      dispatcher.requestViewFocusChange(
+        viewId: view.viewId,
+        state: ui.ViewFocusState.focused,
+        direction: ui.ViewFocusDirection.forward,
+      );
+
+      expect(domDocument.activeElement, input);
+
+      expect(dispatchedViewFocusEvents, hasLength(1));
+
+      expect(dispatchedViewFocusEvents[0].viewId, view.viewId);
+      expect(dispatchedViewFocusEvents[0].state, ui.ViewFocusState.focused);
+      expect(dispatchedViewFocusEvents[0].direction, ui.ViewFocusDirection.forward);
+    });
   });
 }