Make Sky example game run offline

sky/sdk/lib/mojo/net/fetch.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:typed_data';
 
 import 'package:mojo/core.dart' as core;
 import 'package:mojom/mojo/network_service.mojom.dart';
 import 'package:mojom/mojo/url_loader.mojom.dart';
@@ skipping 40 matching lines @@
   return fetch(request);
 }
 
 Future<Response> fetchBody(String relativeUrl) async {
   UrlResponse response = await fetchUrl(relativeUrl);
   if (response.body == null) return new Response(null);
 
   ByteData data = await core.DataPipeDrainer.drainHandle(response.body);
   return new Response(data);
 }
+
+Future<String> fetchString(String relativeUrl) async {
+  Response response = await fetchBody(relativeUrl);
+  return response.bodyAsString();
+}