Make the unittest's breath test a bit more resilient against timing issue.

pkg/unittest/test/breath_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 unittest.breath_test;
 
 import 'dart:async';
 
 import 'package:unittest/unittest.dart';
 
 void main() {
   // Test the sync test 'breath' feature of unittest.
 
+  // We use the testStartStopwatch to determine if the 'starve'
+  // test was executed within a small enough time interval from
+  // the first test that we are guaranteed the second test is
+  // running in a microtask. If the second test is running as a
+  // microtask we are guaranteed the timer scheduled in the
+  // first test has not been run yet.
+  var testStartStopwatch = new Stopwatch()..start();
+
   group('breath', () {
     var sentinel = 0;
-    var start;
 
     test('initial', () {
       Timer.run(() {
         sentinel = 1;
       });
     });
 
     test('starve', () {
-      start = new DateTime.now().millisecondsSinceEpoch;
-      var now;
-      do {
+      // If less than BREATH_INTERVAL time has passed since before
+      // we started the test group then the previous test's timer
+      // has not been run (at least this is what we are testing).
+      if (testStartStopwatch.elapsed.inMilliseconds <= BREATH_INTERVAL) {
         expect(sentinel, 0);
-        now = new DateTime.now().millisecondsSinceEpoch;
-      } while (now - start <= BREATH_INTERVAL);
+      }
+
+      // Next we wait for at least BREATH_INTERVAL to guaranteed the
+      // next (third) test is run using a timer which means it will
+      // run after the timer scheduled in the first test and hence
+      // the sentinel should have been set to 1.
+      var sw = new Stopwatch()..start();
+      while (sw.elapsed.inMilliseconds < BREATH_INTERVAL);
     });
 
     test('breathed', () {
       expect(sentinel, 1);
     });
   });
 }