Enables clean VM shutdown.

tests/isolate/nested_spawn_test.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Dart test program for testing that isolates can spawn other isolates.
 
 library NestedSpawnTest;
 import 'dart:isolate';
 import 'package:unittest/unittest.dart';
 import "remote_unittest_helper.dart";
 
 void isolateA(message) {
   message.add("isolateA");
   Isolate.spawn(isolateB, message);
 }
 
 void isolateB(message) {
   message.add("isolateB");
   message[0].send(message);
 }
 
 void main([args, port]) {
   if (testRemote(main, port)) return;
   test("spawned isolates can spawn nested isolates", () {
     ReceivePort port = new ReceivePort();
     Isolate.spawn(isolateA, [port.sendPort, "main"]);
-    port.first.then((message) {
+    return port.first.then((message) {
       expect("main", message[1]);
       expect("isolateA", message[2]);
       expect("isolateB", message[3]);
     });
   });
 }