Fix printing of dates with year 0.

tests/corelib/date_time8_test.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.
+
+// Make sure the year 0 is correctly printed.
+
+testUtc() {
+  var d = new Date(0, 1, 1, isUtc: true);
+  Expect.equals("0000-01-01 00:00:00.000Z", d.toString());
+}
+
+testLocal() {
+  var d = new Date(0, 1, 1);
+  Expect.equals("0000-01-01 00:00:00.000", d.toString());
+}
+
+main() {
+  testUtc();
+  testLocal();
+}