Reland: Migration to PlatformDispatcher and multi-window

ci/licenses_golden/licenses_flutter

@@ -371,6 +371,7 @@ FILE: ../../../flutter/lib/ui/painting/single_frame_codec.h
 FILE: ../../../flutter/lib/ui/painting/vertices.cc
 FILE: ../../../flutter/lib/ui/painting/vertices.h
 FILE: ../../../flutter/lib/ui/painting/vertices_unittests.cc
+FILE: ../../../flutter/lib/ui/platform_dispatcher.dart
 FILE: ../../../flutter/lib/ui/plugins.dart
 FILE: ../../../flutter/lib/ui/plugins/callback_cache.cc
 FILE: ../../../flutter/lib/ui/plugins/callback_cache.h
@@ -500,6 +501,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/keyboard.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/mouse_cursor.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/onscreen_logging.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/picture.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/platform_dispatcher.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/platform_views.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/plugins.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/pointer_binding.dart
@@ -554,6 +556,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/ui/natives.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/painting.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/path.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/path_metrics.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/ui/platform_dispatcher.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/pointer.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/semantics.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/ui/test_embedding.dart

lib/ui/compositing.dart

@@ -10,8 +10,8 @@ part of dart.ui;
 ///
 /// To create a Scene object, use a [SceneBuilder].
 ///
-/// Scene objects can be displayed on the screen using the
-/// [Window.render] method.
+/// Scene objects can be displayed on the screen using the [FlutterView.render]
+/// method.
 @pragma('vm:entry-point')
 class Scene extends NativeFieldWrapperClass2 {
   /// This class is created by the engine, and should not be instantiated
@@ -186,7 +186,7 @@ class PhysicalShapeEngineLayer extends _EngineLayerWrapper {
 
 /// Builds a [Scene] containing the given visuals.
 ///
-/// A [Scene] can then be rendered using [Window.render].
+/// A [Scene] can then be rendered using [FlutterView.render].
 ///
 /// To draw graphical operations onto a [Scene], first create a
 /// [Picture] using a [PictureRecorder] and a [Canvas], and then add
@@ -655,13 +655,13 @@ class SceneBuilder extends NativeFieldWrapperClass2 {
   ///  - 0x08: visualizeEngineStatistics - graph UI thread frame times
   /// Set enabledOptions to 0x0F to enable all the currently defined features.
   ///
-  /// The "UI thread" is the thread that includes all the execution of
-  /// the main Dart isolate (the isolate that can call
-  /// [Window.render]). The UI thread frame time is the total time
-  /// spent executing the [Window.onBeginFrame] callback. The "raster
-  /// thread" is the thread (running on the CPU) that subsequently
-  /// processes the [Scene] provided by the Dart code to turn it into
-  /// GPU commands and send it to the GPU.
+  /// The "UI thread" is the thread that includes all the execution of the main
+  /// Dart isolate (the isolate that can call [FlutterView.render]). The UI
+  /// thread frame time is the total time spent executing the
+  /// [PlatformDispatcher.onBeginFrame] callback. The "raster thread" is the
+  /// thread (running on the CPU) that subsequently processes the [Scene]
+  /// provided by the Dart code to turn it into GPU commands and send it to the
+  /// GPU.
   ///
   /// See also the [PerformanceOverlayOption] enum in the rendering library.
   /// for more details.
@@ -802,7 +802,7 @@ class SceneBuilder extends NativeFieldWrapperClass2 {
   ///
   /// Returns a [Scene] containing the objects that have been added to
   /// this scene builder. The [Scene] can then be displayed on the
-  /// screen with [Window.render].
+  /// screen with [FlutterView.render].
   ///
   /// After calling this function, the scene builder object is invalid and
   /// cannot be used further.

lib/ui/compositing/scene.cc

@@ -42,9 +42,10 @@ Scene::Scene(std::shared_ptr<flutter::Layer> rootLayer,
              uint32_t rasterizerTracingThreshold,
              bool checkerboardRasterCacheImages,
              bool checkerboardOffscreenLayers) {
+  // Currently only supports a single window.
   auto viewport_metrics = UIDartState::Current()
                               ->platform_configuration()
-                              ->window()
+                              ->get_window(0)
                               ->viewport_metrics();
 
   layer_tree_ = std::make_unique<LayerTree>(

lib/ui/dart_ui.gni

@@ -13,6 +13,7 @@ dart_ui_files = [
   "//flutter/lib/ui/lerp.dart",
   "//flutter/lib/ui/natives.dart",
   "//flutter/lib/ui/painting.dart",
+  "//flutter/lib/ui/platform_dispatcher.dart",
   "//flutter/lib/ui/plugins.dart",
   "//flutter/lib/ui/pointer.dart",
   "//flutter/lib/ui/semantics.dart",