Use PrivateDart for ImageDecoder constructors

packages/flutter/lib/src/services/asset_bundle.dart

@@ -70,8 +70,7 @@ class MojoAssetBundle extends AssetBundle {
     return _imageCache.putIfAbsent(key, () {
       Completer<sky.Image> completer = new Completer<sky.Image>();
       load(key).then((assetData) {
-        new sky.ImageDecoder(completer.complete)
-               ..initWithConsumer(assetData.handle.h);
+        new sky.ImageDecoder.consume(assetData.handle.h, completer.complete);
       });
       return new ImageResource(completer.future);
     });

packages/flutter/lib/src/services/image_cache.dart

@@ -23,8 +23,7 @@ class _ImageCache {
           print("Failed (${response.statusCode}) to load image ${url}");
           completer.complete(null);
         } else {
-          new sky.ImageDecoder(completer.complete)
-                 ..initWithConsumer(response.body.handle.h);
+          new sky.ImageDecoder.consume(response.body.handle.h, completer.complete);
         }
       });
       return new ImageResource(completer.future);