Fix bugs found by Dart analyzer

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:sky.internals' as internals;
 import 'dart:typed_data';
 
 import 'package:mojo/core.dart' as core;
@@ skipping 35 matching lines @@
 
 class MojoAssetBundle extends AssetBundle {
   MojoAssetBundle(AssetBundleProxy this._bundle);
 
   factory MojoAssetBundle.fromNetwork(String relativeUrl) {
     AssetBundleProxy bundle = new AssetBundleProxy.unbound();
     _fetchAndUnpackBundle(relativeUrl, bundle);
     return new MojoAssetBundle(bundle);
   }
 
-  final AssetBundleProxy _bundle;
+  AssetBundleProxy _bundle;
   Map<String, Future<sky.Image>> _imageCache = new Map<String, Future<sky.Image>>();
   Map<String, Future<String>> _stringCache = new Map<String, Future<String>>();
 
   void close() {
     _bundle.close();
     _bundle = null;
     _imageCache = null;
   }
 
   Future<sky.Image> loadImage(String key) {
@@ skipping 21 matching lines @@
   try {
     AssetBundleProxy bundle = new AssetBundleProxy.fromHandle(
         new core.MojoHandle(internals.takeRootBundleHandle()));
     return new MojoAssetBundle(bundle);
   } catch (e) {
     return null;
   }
 }
 
 final AssetBundle rootBundle = _initRootBundle();