Dart: Fix bug in unbind() for Stubs.

mojo/dart/apptests/dart_apptests/lib/src/echo_apptests.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.
 
 library echo_apptests;
 
 import 'dart:async';
 
 import 'package:mojo_apptest/apptest.dart';
 import 'package:mojo/application.dart';
@@ skipping 68 matching lines @@
           "quit", milliseconds)).then((result) {
         fail('This future should not complete.');
       }, onError: (e) {
         expect(e is ProxyError, isTrue);
       });
 
       return new Future.delayed(
           new Duration(milliseconds: 10), () => echoProxy.close());
     });
 
+    test('Swap', () async {
+      var echoProxy =
+          new EchoServiceProxy.connectToService(application, "mojo:dart_echo");
+
+      for (int i = 0; i < 10; i++) {
+        var v =
+            await echoProxy.responseOrError(echoProxy.ptr.echoString("foo"));
+        expect(v.value, equals("foo"));
+      }
+
+      echoProxy.impl.errorFuture.then((e) {
+        fail("echoProxy: $e");
+      });
+
+      // Trigger an implementation swap in the echo server.
+      echoProxy.ptr.swap();
+
+      expect(echoProxy.impl.isBound, isTrue);
+
+      for (int i = 0; i < 10; i++) {
+        var v =
+            await echoProxy.responseOrError(echoProxy.ptr.echoString("foo"));
+        expect(v.value, equals("foo"));
+      }
+
+      var q = await echoProxy.responseOrError(echoProxy.ptr.echoString("quit"));
+      expect(q.value, equals("quit"));
+
+      await echoProxy.close();
+    });
+
     test('Multiple Error Checks Success', () {
       var echoProxy =
           new EchoServiceProxy.connectToService(application, "mojo:dart_echo");
 
       List<Future> futures = [];
       for (int i = 0; i < 100; i++) {
         var f = echoProxy.responseOrError(echoProxy.ptr.echoString("foo"))
                          .then((r) {
           expect(r.value, equals("foo"));
         }, onError: (e) {
@@ skipping 179 matching lines @@
       await echoProxy.close();
 
       r = await differentEchoProxy.responseOrError(
           differentEchoProxy.ptr.echoString("foo"));
       expect(r.value, equals("foo"));
 
       await differentEchoProxy.close();
     });
   });
 }