Fix bug in VM timer implementation.

tests/lib/async/run_async6_test.dart

Index: tests/lib/async/run_async6_test.dart
diff --git a/tests/lib/async/run_async_test.dart b/tests/lib/async/run_async6_test.dart
similarity index 63%
copy from tests/lib/async/run_async_test.dart
copy to tests/lib/async/run_async6_test.dart
index 0ec7afae1fa6b2c289bc09a3c4fe067f382c0896..e408b30bd70eecb8847f4496ff8fa8c84e744c5d 100644
--- a/tests/lib/async/run_async_test.dart
+++ b/tests/lib/async/run_async6_test.dart
@@ -7,8 +7,16 @@ 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');
+    });
   });
 }