Rename Date to DateTime.

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.utc(0, 1, 1);
+  var d = new DateTime.utc(0, 1, 1);
   Expect.equals("0000-01-01 00:00:00.000Z", d.toString());
 }
 
 testLocal() {
-  var d = new Date(0, 1, 1);
+  var d = new DateTime(0, 1, 1);
   Expect.equals("0000-01-01 00:00:00.000", d.toString());
 }
 
 main() {
   testUtc();
   testLocal();
 }