Move timer tests to async tests.

tests/isolate/timer_cancel2_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.
-
-library timer_cancel2_test;
-
-import 'dart:async';
-import '../../pkg/unittest/lib/unittest.dart';
-
-main() {
-  // Test that a timeout handler can cancel itself.
-  test("timer cancel test 2", () {
-    var cancelTimer;
-
-    void cancelHandler(Timer timer) {
-      cancelTimer.cancel();
-    }
-
-    cancelTimer = new Timer.periodic(const Duration(milliseconds: 1),
-                                     expectAsync1(cancelHandler));
-  });
-}