Fix date_time_test to not assume a particular slowness of running code.

tests/corelib/date_time_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.
 
 import "package:expect/expect.dart";
 
 // Dart test program for DateTime.
 
 // Identical to _MAX_MILLISECONDS_SINCE_EPOCH in date_time.dart
 const int _MAX_MILLISECONDS = 8640000000000000;
 
 // Tests if the time moves eventually forward.
 void testNow() {
   var t1 = new DateTime.now();
   bool timeMovedForward = false;
-  for (int i = 0; i < 1000000; i++) {
-    var t2 = new DateTime.now();
-    if (t1.millisecondsSinceEpoch < t2.millisecondsSinceEpoch) {
-      timeMovedForward = true;
-      break;
+  const int N = 1000000;
+  outer: while (true) {
+    for (int i = N; i > 0; i--) {
+      var t2 = new DateTime.now();
+      if (t1.millisecondsSinceEpoch < t2.millisecondsSinceEpoch) {
+        break outer;
+      }
     }
+    print("testNow: No Date.now() progress in $N loops. Time: $t1");
   }
-  Expect.equals(true, timeMovedForward);
   Expect.isFalse(t1.isUtc);
 }
 
 void testValue() {
   var dt1 = new DateTime.now();
   var millisecondsSinceEpoch = dt1.millisecondsSinceEpoch;
   var dt2 = new DateTime.fromMillisecondsSinceEpoch(millisecondsSinceEpoch);
   Expect.equals(millisecondsSinceEpoch, dt2.millisecondsSinceEpoch);
 }
 
@@ skipping 928 matching lines @@
   testSubAdd();
   testUnderflowAndOverflow();
   testDateStrings();
   testEquivalentYears();
   testExtremes();
   testFarAwayDates();
   testWeekday();
   testToStrings();
   testIsoString();
 }