Use Stopwatch instead of DateTime to measure elapsed time in tests.

tests/lib/async/timer_test.dart

 // Copyright (c) 2011, 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_test;
 
 import 'dart:async';
 import 'package:unittest/unittest.dart';
 
 const int STARTTIMEOUT = 1050;
 const int DECREASE = 200;
 const int ITERATIONS = 5;
 
-int startTime;
+Stopwatch stopwatch = new Stopwatch();
 int timeout;
 int iteration;
 
 // Some browsers (Firefox and IE so far) can trigger too early. Add a safety
 // margin. We use identical(1, 1.0) as an easy way to know if the test is
 // compiled by dart2js.
 int get safetyMargin => identical(1, 1.0) ? 100 : 0;
 
 void timeoutHandler() {
-  int endTime = (new DateTime.now()).millisecondsSinceEpoch;
-  expect(endTime - startTime + safetyMargin, greaterThanOrEqualTo(timeout));
+  expect(stopwatch.elapsedMilliseconds + safetyMargin,
+         greaterThanOrEqualTo(timeout));
   if (iteration < ITERATIONS) {
     iteration++;
     timeout = timeout - DECREASE;
     Duration duration = new Duration(milliseconds: timeout);
-    startTime = (new DateTime.now()).millisecondsSinceEpoch;
+    stopwatch.reset();
     new Timer(duration, expectAsync(timeoutHandler));
   }
 }
 
 main() {
   test("timeout test", () {
     iteration = 0;
     timeout = STARTTIMEOUT;
     Duration duration = new Duration(milliseconds: timeout);
-    startTime = (new DateTime.now()).millisecondsSinceEpoch;
+    stopwatch.start();
     new Timer(duration, expectAsync(timeoutHandler));
   });
 }