Fix bug in VM timer implementation.

tests/lib/async/run_async6_test.dart

 // Copyright (c) 2013, 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.
 
 library run_async_test;
 
 import 'dart:async';
 import '../../../pkg/unittest/lib/unittest.dart';
 
+bool _unhandledExceptionCallback(exception) => true;
+
 main() {
   test("run async test", () {
-    runAsync(expectAsync0(() {}));
+    var callback = expectAsync0(() {});
+    runAsync(() {
+      runAsync(() {
+        callback();
+      });
+      throw new Exception('exception');
+    });
   });
 }