Rework Timer interface.

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 int TIMEOUT = 500;
+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.repeating(TIMEOUT, 
         expectAsync1(timeoutHandler, count: ITERATIONS));
   });
 }