Clean VM thread shutdown:

tests/isolate/nested_spawn2_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 and
 // that the nested isolates can communicate with the main once the spawner has
 // disappeared.
 
 library NestedSpawn2Test;
 import 'dart:isolate';
@@ skipping 39 matching lines @@
       }));
     }));
   }));
 }
 
 void main([args, port]) {
   if (testRemote(main, port)) return;
   test("spawned isolate can spawn other isolates", () {
     ReceivePort init = new ReceivePort();
     Isolate.spawn(isolateA, init.sendPort);
-    init.first.then(expectAsync((port) {
+    return init.first.then(expectAsync((port) {
       _call(port, "launch nested!", expectAsync((msg, replyTo) {
         expect(msg[0], "0");
         _call(replyTo, msg1, expectAsync((msg, replyTo) {
           expect(msg[0], "2");
           _call(replyTo, msg3, expectAsync((msg, replyTo) {
             expect(msg[0], "4");
             _call(replyTo, msg5, expectAsync((msg, _) {
               expect(msg[0], "6");
             }));
           }));
         }));
       }));
     }));
   });
 }