Move image loading out of C++ into Dart

sky/engine/core/core.gni

Index: sky/engine/core/core.gni
diff --git a/sky/engine/core/core.gni b/sky/engine/core/core.gni
index 0a53f4630e42492853fb57b1a99a00bdbd9379d2..464147fce28d349ffe0c01b08697bc4bbe6fbd49 100644
--- a/sky/engine/core/core.gni
+++ b/sky/engine/core/core.gni
@@ -798,8 +798,8 @@ sky_core_files = [
   "inspector/ScriptAsyncCallStack.h",
   "inspector/ScriptGCEventListener.h",
   "layout/LayoutCallback.h",
-  "loader/CanvasImageLoader.cpp",
-  "loader/CanvasImageLoader.h",
+  "loader/CanvasImageDecoder.cpp",
+  "loader/CanvasImageDecoder.h",
   "loader/DocumentLoadTiming.cpp",
   "loader/DocumentLoadTiming.h",
   "loader/EmptyClients.cpp",
@@ -812,7 +812,7 @@ sky_core_files = [
   "loader/FrameLoaderTypes.h",
   "loader/ImageLoader.cpp",
   "loader/ImageLoader.h",
-  "loader/ImageLoaderCallback.h",
+  "loader/ImageDecoderCallback.h",
   "loader/MojoLoader.cpp",
   "loader/MojoLoader.h",
   "loader/NavigationPolicy.h",
@@ -1142,8 +1142,8 @@ core_idl_files = get_path_info([
                                  "html/TextMetrics.idl",
                                  "html/VoidCallback.idl",
                                  "layout/LayoutCallback.idl",
-                                 "loader/ImageLoader.idl",
-                                 "loader/ImageLoaderCallback.idl",
+                                 "loader/ImageDecoder.idl",
+                                 "loader/ImageDecoderCallback.idl",
                                  "painting/Canvas.idl",
                                  "painting/ColorFilter.idl",
                                  "painting/DrawLooper.idl",