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

tests/lib/async/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 'package:unittest/unittest.dart';
 
 const Duration TIMEOUT = const Duration(milliseconds: 500);
 const int ITERATIONS = 5;
 
 Timer timer;
-int startTime;
+Stopwatch stopwatch = new Stopwatch();
 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(Timer timer) {
-  int endTime = (new DateTime.now()).millisecondsSinceEpoch;
   iteration++;
-  if (iteration < ITERATIONS) {
-    startTime = (new DateTime.now()).millisecondsSinceEpoch;
-  } else {
-    expect(iteration, ITERATIONS);
+  expect(iteration, lessThanOrEqualTo(ITERATIONS));
+  if (iteration == ITERATIONS) {
+    // When we are done with all of the iterations, we expect a
+    // certain amount of time to have passed.  Checking the time on
+    // each iteration doesn't work because the timeoutHandler runs
+    // concurrently with the periodic timer.
+    expect(stopwatch.elapsedMilliseconds + safetyMargin,
+           greaterThanOrEqualTo(ITERATIONS * TIMEOUT.inMilliseconds));
     timer.cancel();
   }
 }
 
 main() {
   test("timer_repeat", () {
     iteration = 0;
-    startTime = new DateTime.now().millisecondsSinceEpoch;
+    stopwatch.start();
     timer = new Timer.periodic(TIMEOUT,
         expectAsync(timeoutHandler, count: ITERATIONS));
   });
 }