Dart: Fix bug in unbind() for Stubs.

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

 // Copyright 2016 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';
 
 /// DummyEchoServiceImpl is unused in the corresponding test_app.
 class DummyEchoServiceImpl implements EchoService {
   dynamic echoString(String value, [Function responseFactory]) => null;
 
   dynamic delayedEchoString(String value, int millis,
       [Function responseFactory]) => null;
 
+  void swap() {}
+
   void quit() {}
 }
 
 class EchoApplication extends Application {
   EchoApplication.fromHandle(MojoHandle handle) : super.fromHandle(handle);
 
   @override
   void acceptConnection(String requestorUrl, String resolvedUrl,
       ApplicationConnection connection) {
     connection.provideService(EchoService.serviceName,
       (endpoint) => new DummyEchoServiceImpl(),
       description: EchoServiceStub.serviceDescription);
   }
 }
 
 main(List args, Object handleToken) {
   MojoHandle appHandle = new MojoHandle(handleToken);
   new EchoApplication.fromHandle(appHandle)
     ..onError = ((_) {
       MojoHandle.reportLeakedHandles();
     });
 }