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

pkg/intl/test/date_time_format_test_core.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.
 
 /**
  * Tests the DateFormat library in dart. This file contains core tests that
  * are run regardless of where the locale data is found, so it doesn't expect to
  * be run on its own, but rather to be imported and run from another test file.
  */
 
@@ skipping 126 matching lines @@
   }
 }
 
 testRoundTripParsing(String localeName, DateTime date) {
   // In order to test parsing, we can't just read back the date, because
   // printing in most formats loses information. But we can test that
   // what we parsed back prints the same as what we originally printed.
   // At least in most cases. In some cases, we can't even do that. e.g.
   // the skeleton WEEKDAY can't be reconstructed at all, and YEAR_MONTH
   // formats don't give us enough information to construct a valid date.
-  var badSkeletons = [
+  var badSkeletons = const [
       DateFormat.ABBR_WEEKDAY,
       DateFormat.WEEKDAY,
       DateFormat.QUARTER,
       DateFormat.ABBR_QUARTER,
       DateFormat.YEAR,
       DateFormat.YEAR_NUM_MONTH,
       DateFormat.YEAR_ABBR_MONTH,
       DateFormat.YEAR_MONTH,
       DateFormat.MONTH_WEEKDAY_DAY,
       DateFormat.NUM_MONTH_WEEKDAY_DAY,
       DateFormat.ABBR_MONTH_WEEKDAY_DAY];
+  var originalTime = new DateTime.now();
+  var originalTimeZoneOffset = date.timeZoneOffset;
+  var originalTimeZoneName = date.timeZoneName;
   for(int i = 0; i < formatsToTest.length; i++) {
     var skeleton = formatsToTest[i];
     if (!badSkeletons.any((x) => x == skeleton)) {
       var format = new DateFormat(skeleton, localeName);
       var actualResult = format.format(date);
       var parsed = format.parse(actualResult);
       var thenPrintAgain = format.format(parsed);
+      // We've seen a case where this failed in a way that seemed like a time
+      // zone shifting or some other strange behaviour that caused an off by
+      // one error in the date. Check for this and print out as much information
+      // as possible if it occurs again.
+      if (thenPrintAgain != actualResult) {
+        print("Date mismatch!");
+        print("  Expected $actualResult");
+        print("  Got $thenPrintAgain");
+        print("  Original date = $date");
+        print("  Original ms = ${date.millisecondsSinceEpoch}");
+        print("  Parsed back to $parsed");
+        print("  Parsed ms = ${parsed.millisecondsSinceEpoch}");
+        print("  Original tz = $originalTimeZoneOffset");
+        print("  Current tz name = $originalTimeZoneName");
+        print("  Current tz = ${parsed.timeZoneOffset}");
+        print("  Current tz name = ${parsed.timeZoneName}");
+        print("  Start time = $originalTime");
+        print("  Current time ${new DateTime.now()}");
+      }
       expect(thenPrintAgain, equals(actualResult));
     }
   }
 }
 
 /** A shortcut for returning all the locales we have available.*/
 List<String> allLocales() => DateFormat.allLocalesWithSymbols();
 
 Function _subsetFunc;
 
@@ skipping 219 matching lines @@
     var f = new DateFormat("D");
     var dates = generateDates(2012, 1);
     var nonLeapDates = generateDates(2013, 0);
     verifyOrdinals(dates);
     verifyOrdinals(nonLeapDates);
     // Check one hard-coded just to be on the safe side.
     var aDate = new DateTime(2012, 4, 27, 13, 58, 59, 012);
     expect(f.format(aDate), "118");
   });
 }