Work around DateTime creation flakiness

pkg/intl/test/date_time_format_test_core.dart

Index: pkg/intl/test/date_time_format_test_core.dart
diff --git a/pkg/intl/test/date_time_format_test_core.dart b/pkg/intl/test/date_time_format_test_core.dart
index 6ac9d75af3c24e0f4a556da239785653dc1b3546..44e75ca28cb65ef22b2b7a876475423f364355eb 100644
--- a/pkg/intl/test/date_time_format_test_core.dart
+++ b/pkg/intl/test/date_time_format_test_core.dart
@@ -373,7 +373,12 @@ void runDateTests(Function subsetFunc) {
    */
   Map<int, DateTime> generateDates(int year, int leapDay) =>
       new Iterable.generate(365 + leapDay, (n) => n + 1)
-        .map((day) => new DateTime(year, 1, day)).toList().asMap();
+        .map((day) {
+          var result = new DateTime(year, 1, day);
+          // TODO(alanknight): This is a workaround for dartbug.com/15560.
+          if (result.toUtc() == result) result = new DateTime(year, 1, day);
+          return result;
+        }).toList().asMap();
 
   void verifyOrdinals(Map dates) {
     var f = new DateFormat("D");