Move timer tests to async tests.

tests/isolate/timer_repeat_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_repeat_test;
-
-import 'dart:async';
-import '../../pkg/unittest/lib/unittest.dart';
-
-const Duration TIMEOUT = const Duration(milliseconds: 500);
-const int ITERATIONS = 5;
-
-Timer timer;
-int startTime;
-int iteration;
-
-void timeoutHandler(Timer timer) {
-  int endTime = (new DateTime.now()).millisecondsSinceEpoch;
-  iteration++;
-  if (iteration < ITERATIONS) {
-    startTime = (new DateTime.now()).millisecondsSinceEpoch;
-  } else {
-    expect(iteration, ITERATIONS);
-    timer.cancel();
-  }
-}
-
-main() {
-  test("timer_repeat", () {
-    iteration = 0;
-    startTime = new DateTime.now().millisecondsSinceEpoch;
-    timer = new Timer.periodic(TIMEOUT,
-        expectAsync1(timeoutHandler, count: ITERATIONS));
-  });
-}