Use a 0-length timer in async test to make jsshell happy.

tests/language/asyncstar_yield_test.dart

 // Copyright (c) 2015, 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.
 
 import "dart:async";
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
 Stream<int> foo1() async* {
   yield 1;
   var p = await new Future.value(10);
   yield p + 10;
 }
 
 Stream<int> foo2() async* {
   int i = 0;
   while (true) {
-    await (new Future.delayed(new Duration(milliseconds: 10), () {}));
+    await (new Future.delayed(new Duration(milliseconds: 0), () {}));
     if (i > 10) return;
     yield i;
     i++;
   }
 }
 
 Stream<int> foo3(p) async* {
   int i = 0;
   bool t = false;
   yield null;
@@ skipping 37 matching lines @@
       await (foo4().take(10).toList()));
   Expect.isTrue(await (finalized.future));
 }
 
 main ()  {
   asyncStart();
   test().then((_) {
     asyncEnd();
   });
 }