Fix a bug found by the 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 'package:mojo/core.dart' as core;
 import 'package:mojom/mojo/asset_bundle/asset_bundle.mojom.dart';
 
@@ skipping 13 matching lines @@
 
   void close() { }
 
   Future<sky.Image> loadImage(String name) {
     Uri url = _baseUrl.resolve(name);
     return image_cache.load(url.toString());
   }
 }
 
 Future _fetchAndUnpackBundle(String relativeUrl, AssetBundleProxy bundle) async {
-  core.MojoDataPipeConsumer bundleData = (await fetchUrl(url)).body;
+  core.MojoDataPipeConsumer bundleData = (await fetchUrl(relativeUrl)).body;
   AssetUnpackerProxy unpacker = new AssetUnpackerProxy.unbound();
   shell.requestService("mojo:asset_bundle", unpacker);
   unpacker.ptr.unpackZipStream(bundleData, bundle);
   unpacker.close();
 }
 
 class MojoAssetBundle extends AssetBundle {
   MojoAssetBundle(AssetBundleProxy this._bundle);
 
   factory MojoAssetBundle.fromNetwork(String relativeUrl) {
@@ skipping 14 matching lines @@
   Future<sky.Image> loadImage(String name) {
     return _imageCache.putIfAbsent(name, () {
       Completer<sky.Image> completer = new Completer<sky.Image>();
       _bundle.ptr.getAsStream(name).then((response) {
         new sky.ImageDecoder(response.assetData.handle.h, completer.complete);
       });
       return completer.future;
     });
   }
 }