Convert image_cache to using Futures rather than callbacks

sky/sdk/lib/mojo/asset_bundle.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:sky' as sky;
 
 import 'package:mojo/core.dart' as core;
 import 'package:mojom/mojo/asset_bundle/asset_bundle.mojom.dart';
 
 import 'shell.dart' as shell;
 import 'net/fetch.dart';
 
 Future<sky.Image> _decodeImage(core.MojoDataPipeConsumer assetData) {
-  Completer<sky.Image> completer = new Completer();
+  Completer<sky.Image> completer = new Completer<sky.Image>();
   new sky.ImageDecoder(assetData.handle.h, completer.complete);
   return completer.future;
 }
 
 abstract class AssetBundle {
   void close();
   Future<sky.Image> fetchImage(String key);
 }
 
 class NetworkAssetBundle extends AssetBundle {
@@ skipping 33 matching lines @@
     _bundle.close();
     _bundle = null;
   }
 
   Future<sky.Image> fetchImage(String name) async {
     core.MojoDataPipeConsumer assetData =
         (await _bundle.ptr.getAsStream(name)).assetData;
     return await _decodeImage(assetData);
   }
 }