Added Sky Mojo embedder singleton, restored network test

sky/framework/shell.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 "/mojo/public/dart/application.dart";
-import "dart:mojo_bindings" as bindings;
-import "dart:mojo_core" as core;
-import "dart:sky.internals" as internals;
-import "package:mojo/public/interfaces/application/service_provider.mojom.dart";
-import "package:mojo/public/interfaces/application/shell.mojom.dart";
-
-final ShellProxy _shell = new ShellProxy.fromHandle(
-    new core.MojoHandle(internals.takeShellProxyHandle()));
-
-ApplicationConnection connectToApplication(String url) {
-  var serviceProviderProxy = new ServiceProviderProxy.unbound();
-  _shell.connectToApplication(url, serviceProviderProxy, null);
-  return new ApplicationConnection(null, serviceProviderProxy);
-}
-
-void connectToService(String url, bindings.Proxy proxy) {
-  connectToApplication(url).requestService(proxy);
-}