Create an ImageHandle wrapper (flutter#21057)

Allows for reference counting of images before disposal.

This will allow multiple callers to hold a reference to an image and dispose of their reference without disposing the underlying image until all handles have been disposed.

This will be used by the framework to help resolve some of the kludge I was trying to introduce in flutter#64582

Author: dnfield

ci/licenses_golden/licenses_flutter

@@ -328,8 +328,6 @@ FILE: ../../../flutter/lib/ui/painting/color_filter.cc
 FILE: ../../../flutter/lib/ui/painting/color_filter.h
 FILE: ../../../flutter/lib/ui/painting/engine_layer.cc
 FILE: ../../../flutter/lib/ui/painting/engine_layer.h
-FILE: ../../../flutter/lib/ui/painting/frame_info.cc
-FILE: ../../../flutter/lib/ui/painting/frame_info.h
 FILE: ../../../flutter/lib/ui/painting/gradient.cc
 FILE: ../../../flutter/lib/ui/painting/gradient.h
 FILE: ../../../flutter/lib/ui/painting/image.cc

lib/ui/BUILD.gn

@@ -30,8 +30,6 @@ source_set("ui") {
     "painting/color_filter.h",
     "painting/engine_layer.cc",
     "painting/engine_layer.h",
-    "painting/frame_info.cc",
-    "painting/frame_info.h",
     "painting/gradient.cc",
     "painting/gradient.h",
     "painting/image.cc",

lib/ui/compositing.dart

@@ -23,14 +23,21 @@ class Scene extends NativeFieldWrapperClass2 {
 
   /// Creates a raster image representation of the current state of the scene.
   /// This is a slow operation that is performed on a background thread.
+  ///
+  /// Callers must dispose the [Image] when they are done with it. If the result
+  /// will be shared with other methods or classes, [Image.clone] should be used
+  /// and each handle created must be disposed.
   Future<Image> toImage(int width, int height) {
     if (width <= 0 || height <= 0) {
       throw Exception('Invalid image dimensions.');
     }
-    return _futurize((_Callback<Image> callback) => _toImage(width, height, callback));
+    return _futurize((_Callback<Image> callback) => _toImage(width, height, (_Image image) {
+        callback(Image._(image));
+      }),
+    );
   }
 
-  String _toImage(int width, int height, _Callback<Image> callback) native 'Scene_toImage';
+  String _toImage(int width, int height, _Callback<_Image> callback) native 'Scene_toImage';
 
   /// Releases the resources used by this scene.
   ///

lib/ui/dart_ui.cc

@@ -13,7 +13,6 @@
 #include "flutter/lib/ui/painting/codec.h"
 #include "flutter/lib/ui/painting/color_filter.h"
 #include "flutter/lib/ui/painting/engine_layer.h"
-#include "flutter/lib/ui/painting/frame_info.h"
 #include "flutter/lib/ui/painting/gradient.h"
 #include "flutter/lib/ui/painting/image.h"
 #include "flutter/lib/ui/painting/image_descriptor.h"
@@ -70,7 +69,6 @@ void DartUI::InitForGlobal() {
     DartRuntimeHooks::RegisterNatives(g_natives);
     EngineLayer::RegisterNatives(g_natives);
     FontCollection::RegisterNatives(g_natives);
-    FrameInfo::RegisterNatives(g_natives);
     ImageDescriptor::RegisterNatives(g_natives);
     ImageFilter::RegisterNatives(g_natives);
     ImageShader::RegisterNatives(g_natives);