Implement a simple image cache for Sky in Dart (no eviction)

sky/engine/core/loader/CanvasImageLoader.h

Index: sky/engine/core/loader/CanvasImageLoader.h
diff --git a/sky/engine/core/loader/CanvasImageLoader.h b/sky/engine/core/loader/CanvasImageLoader.h
index 63db7656574dcd1fb48abf8954652125c9823579..a8f255b0341f005daa63cc97972fffb97037a00b 100644
--- a/sky/engine/core/loader/CanvasImageLoader.h
+++ b/sky/engine/core/loader/CanvasImageLoader.h
@@ -28,8 +28,6 @@ class CanvasImageLoader : public MojoFetcher::Client,
   }
   virtual ~CanvasImageLoader();
 
-  void load();
-
   // MojoFetcher::Client
   void OnReceivedResponse(mojo::URLResponsePtr) override;
 
@@ -43,8 +41,6 @@ class CanvasImageLoader : public MojoFetcher::Client,
   OwnPtr<MojoFetcher> fetcher_;
   OwnPtr<mojo::common::DataPipeDrainer> drainer_;
   RefPtr<SharedBuffer> buffer_;
-
-  KURL url_;
   OwnPtr<ImageLoaderCallback> callback_;
 };