Add a dateSymbols getter to DateFormat, useful to get e.g. the list of weekday names

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 7e007e45bd3824ff4b21225eb434e45cee7a8cf6..d4a03c1d0dcffc5743f2623599896be413f62ec4 100644
--- a/pkg/intl/test/date_time_format_test_core.dart
+++ b/pkg/intl/test/date_time_format_test_core.dart
@@ -349,4 +349,10 @@ void runDateTests(Function subsetFunc) {
     var knownResult = knownDefault.format(someDate);
     expect(result, knownResult);
   });
+
+  test('Get symbols', () {
+    var emptyFormat = new DateFormat(null, "en_US");
+    var symbols = emptyFormat.dateSymbols;
+    expect(symbols.NARROWWEEKDAYS, ['S', 'M', 'T', 'W', 'T', 'F', 'S']);
+  });
 }