Update references to Mojo core and bindings

sky/framework/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 '../shell.dart' as shell;
 import 'dart:async';
-import 'dart:mojo.core' as core;
 import 'dart:sky' as sky;
 import 'dart:typed_data';
+import 'package:mojo/public/dart/core.dart' as core;
 import 'package:mojo/services/network/public/interfaces/network_service.mojom.dart';
 import 'package:mojo/services/network/public/interfaces/url_loader.mojom.dart';
 
 class Response {
   ByteData body;
 
   Response(this.body);
 
   String bodyAsString() {
     return new String.fromCharCodes(new Uint8List.view(body.buffer));
@@ skipping 16 matching lines @@
 
   loader.close();
   net.close();
 
   if (response.body == null)
     return new Response(null);
 
   ByteData data = await core.DataPipeDrainer.drainHandle(response.body);
   return new Response(data);
 }