improve DateTime parsing a UTC timezone offset

tests/corelib/date_time_parse_test.dart

Index: tests/corelib/date_time_parse_test.dart
diff --git a/tests/corelib/date_time_parse_test.dart b/tests/corelib/date_time_parse_test.dart
index 9f0ad7603753e7d2410c0a575ebb9fba44326252..a8b9827d13a35cb2dcc0c1bb392bbbe4db043d59 100644
--- a/tests/corelib/date_time_parse_test.dart
+++ b/tests/corelib/date_time_parse_test.dart
@@ -19,4 +19,8 @@ main() {
   check(new DateTime(2012, 02, 27), "+20120227");
   check(new DateTime.utc(2012, 02, 27, 14), "2012-02-27T14Z");
   check(new DateTime.utc(-12345, 1, 1), "-123450101 00:00:00 Z");
+  check(new DateTime.utc(2012, 02, 27, 14), "2012-02-27T14+00");
+  check(new DateTime.utc(2012, 02, 27, 14), "2012-02-27T14+0000");
+  check(new DateTime.utc(2012, 02, 27, 14), "2012-02-27T14+00:00");
+  check(new DateTime.utc(2012, 02, 27, 14), "2012-02-27T14 +00:00");
 }