Use Date.getTime instead of Date.getMilliseconds in dart:js.

tests/lib/js/datetime_roundtrip_test.dart

+// Copyright (c) 2014, 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.
+
+import 'dart:js';
+import 'package:expect/expect.dart';
+
+main() {
+  var dt = new DateTime.now();
+  var jsArray = new JsObject.jsify([dt]);
+  var roundTripped = jsArray[0];
+  Expect.isTrue(roundTripped is DateTime);
+  Expect.equals(dt.millisecondsSinceEpoch, roundTripped.millisecondsSinceEpoch);
+}