Dart: Fix bug in unbind() for Stubs.

mojo/dart/apptests/test_apps/echo/lib/main.dart

 // Copyright 2014 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 'package:mojo/application.dart';
 import 'package:mojo/bindings.dart';
 import 'package:mojo/core.dart';
 import 'package:_mojo_for_test_only/test/echo_service.mojom.dart';
@@ skipping 15 matching lines @@
 
   dynamic delayedEchoString(String value, int millis,
       [Function responseFactory]) {
     if (value == "quit") {
       _stub.close();
     }
     return new Future.delayed(
         new Duration(milliseconds: millis), () => responseFactory(value));
   }
 
+  void swap() {
+    _swapImpls(this);
+  }
+
   void quit() {}
+
+  static void _swapImpls(EchoServiceImpl impl) {
+    final stub = impl._stub;
+    final app = impl._application;
+    // It is not allowed to do an unbind in the midst of handling an event, so
+    // it is delayed until popping back out to the event loop.
+    Timer.run(() {
+      final endpoint = stub.unbind();
+      new EchoServiceImpl(app, endpoint);
+    });
+  }
 }
 
 class EchoApplication extends Application {
   EchoApplication.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   @override
   void acceptConnection(String requestorUrl, String resolvedUrl,
       ApplicationConnection connection) {
     // No services are required from the remote end.
     connection.remoteServiceProvider.close();
     connection.provideService(EchoService.serviceName,
         (endpoint) => new EchoServiceImpl(this, endpoint));
   }
 }
 
 main(List args, Object handleToken) {
   MojoHandle appHandle = new MojoHandle(handleToken);
   new EchoApplication.fromHandle(appHandle)
     ..onError = ((_) {
       MojoHandle.reportLeakedHandles();
     });
 }