Migrate engine to use multi-window API

The goal here is to convert from the window singleton to an API that has the concept of multiple windows. I'm not attempting to actually enable creating multiple windows here, just migrate to an API that has a concept of multiple windows.

Author: gspencergoog

ci/licenses_golden/licenses_flutter

@@ -369,6 +369,7 @@ 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
@@ -494,6 +495,7 @@ 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
@@ -548,6 +550,7 @@ 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
@@ -179,7 +179,7 @@ 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
@@ -648,13 +648,13 @@ 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.
@@ -795,7 +795,7 @@ 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",