Make Sky embedder.connection deal with null handles

sky/framework/embedder.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 _EmbedderImpl embedder = new _EmbedderImpl();
 
 class _EmbedderImpl {
-  static final ShellProxy shell = new ShellProxy.fromHandle(
+  ApplicationConnection _connection;
+
+  final ShellProxy shell = new ShellProxy.fromHandle(
       new core.MojoHandle(internals.takeShellProxyHandle()));
 
-  static final ApplicationConnection connection = new ApplicationConnection(
-      new ServiceProviderStub.fromHandle(
-          new core.MojoHandle(internals.takeServicesProvidedToEmbedder())),
-      new ServiceProviderProxy.fromHandle(
-          new core.MojoHandle(internals.takeServicesProvidedByEmbedder())));
+  ApplicationConnection get connection {
+    if (_connection == null) {
+      var stubHandle = new core.MojoHandle(
+          internals.takeServicesProvidedToEmbedder());
+      var proxyHandle = new core.MojoHandle(
+          internals.takeServicesProvidedByEmbedder());
+      _connection = new ApplicationConection(
+         stubHandle.isValid ? ServiceProviderStub.fromHandle(stubHandle)
+             : null;
+         proxyHandle.isValid ? ServiceProviderProxy.fromHandle(proxyHandle)
+             : null;
+      );
+    }
+    return _connection;
+  }
 
   ApplicationConnection connectToApplication(String url) {
     var proxy = new ServiceProviderProxy.unbound();
     var stub = new ServiceProviderStub.unbound();
     shell.connectToApplication(url, proxy, stub);
     return new ApplicationConnection(stub, proxy);
   }
 
   void connectToService(String url, bindings.Proxy proxy) {
     var appSp = new ServiceProviderProxy.unbound();
     shell.connectToApplication(url, appSp, null);
     var pipe = new core.MojoMessagePipe();
     proxy.bind(pipe.endpoints[0]);
     appSp.connectToService(proxy.name, pipe.endpoints[1]);
     appSp.close();
   }
 }