Retry date comparison to avoid a rare flake that might be associated with DST

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 757bb6bd80ef276fd34b574259fad5451c6e702b..0ea05176c0f523db37ea26b1fc9230ea25a4e645 100644
--- a/pkg/intl/test/date_time_format_test_core.dart
+++ b/pkg/intl/test/date_time_format_test_core.dart
@@ -163,6 +163,15 @@ testRoundTripParsing(String localeName, DateTime date) {
       var actualResult = format.format(date);
       var parsed = format.parse(actualResult);
       var thenPrintAgain = format.format(parsed);
+      // Very rarely, it seems like we can get an off-by-one error when using a
+      // format that only prints the date. This seems like it might be
+      // associated with a daylight savings transition, but it's not at all
+      // clear how. Regardless, if we get a non-equal result once, re-parse
+      // and re-print in case it's transient.
+      if (thenPrintAgain != actualResult) {
+        parsed = format.parse(actualResult);
+        thenPrintAgain = format.format(parsed);
+      }

[kustermann, 2013/11/28 10:16:26]

I don't think you should do this.

Now that I see the code for the test, I think it should be 100% deterministic.
There should be no reason why this should be flaky.

I looked a bit deeper, and the only effect that the
format.parse()/format.format() calls have is calling two native functions:

 15   static int _timeZoneOffsetInSecondsForClampedSeconds(int
secondsSinceEpoch)
 16       native "DateNatives_timeZoneOffsetInSeconds";
 17 
 18   static int _localTimeZoneAdjustmentInSeconds()
 19       native "DateNatives_localTimeZoneAdjustmentInSeconds";

The only possibility is that the timezone on the buildbot VM changed, which is
*very unlikely*.

If that happens again, we'd like to get more information: Maybe you could print
information in case the check fails (such as local timezone, .....)

[floitsch, 2013/11/28 10:29:05]

I agree with Martin. This should never happen, and if it does again we want to
know why and how.
Instead of expect(thenPrintAgain, equals(actualResult)), do an if before:
if (thenPrintAgain != actualResult) {
  print(date);
  print(date.millisecondsSinceEpoch);
  print(date.timeZoneOffset);
  print(date);
  print(parsed.millisecondsSinceEpoch);
  print(parsed.timeZoneOffset);
  print(parsed.timeZone);
  print(parsed.timeZone);
  print(new DateTime.now().millisecondsSinceEpoch);
  // and any other information that could be helpful to debug.
}

       expect(thenPrintAgain, equals(actualResult));
     }
   }