Expose the asset_bundle service in SkyShell

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 'dart:typed_data';
 
 import 'package:mojo/core.dart' as core;
 import 'package:mojom/mojo/asset_bundle/asset_bundle.mojom.dart';
@@ skipping 18 matching lines @@
   Future<sky.Image> fetchImage(String path) async {
     core.MojoDataPipeConsumer assetData =
         (await _bundle.ptr.getAsStream(path)).assetData;
     return await _decodeImage(assetData);
   }
 
   AssetBundleProxy _bundle;
 }
 
 Future<AssetBundle> fetchAssetBundle(String url) async {
-  assert(false); // mojo:asset_bundle doesn't exist yet, see https://codereview.chromium.org/1176153007
   core.MojoDataPipeConsumer bundleData = (await fetchUrl(url)).body;
 
   AssetUnpackerProxy unpacker = new AssetUnpackerProxy.unbound();
   shell.requestService("mojo:asset_bundle", unpacker);
   AssetBundleProxy bundle = new AssetBundleProxy.unbound();
   unpacker.ptr.unpackZipStream(bundleData, bundle);
   unpacker.close();
 
   return new AssetBundle(bundle);
 }