Move timer tests to async tests.

tests/isolate/timer_isolate_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 multiple_timer_test;
-
-import 'dart:isolate';
-import 'dart:async';
-import '../../pkg/unittest/lib/unittest.dart';
-
-const Duration TIMEOUT = const Duration(milliseconds: 100);
-
-createTimer() {
-  port.receive((msg, replyTo) {
-    new Timer(TIMEOUT, () {
-      replyTo.send("timer_fired");
-    });
-  });
-}
-
-main() {
-  test("timer in isolate", () {
-    int startTime;
-    int endTime;
-
-    port.receive(expectAsync2((msg, _) {
-      expect("timer_fired", msg);
-      int endTime = (new DateTime.now()).millisecondsSinceEpoch;
-      expect(endTime - startTime, greaterThanOrEqualTo(TIMEOUT.inMilliseconds));
-      port.close();
-    }));
-
-    startTime = (new DateTime.now()).millisecondsSinceEpoch;
-    var sendPort = spawnFunction(createTimer);
-    sendPort.send("sendPort", port.toSendPort());
-  });
-}