Don't change Date to DateTime in JavaScript code.

tools/dom/templates/html/dart2js/impl_Window.darttemplate

 // 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.
 
 part of html;
 
 $(ANNOTATIONS)class $CLASSNAME$EXTENDS$IMPLEMENTS native "@*DOMWindow" {
 
   Document get document => JS('Document', '#.document', this);
 
@@ skipping 84 matching lines @@
 
   @JSName('cancelAnimationFrame')
   void _cancelAnimationFrame(int id) native;
 
   _ensureRequestAnimationFrame() {
     if (JS('bool',
            '!!(#.requestAnimationFrame && #.cancelAnimationFrame)', this, this))
       return;
 
     JS('void',
        r"""

[floitsch, 2013/01/24 16:41:25]

JavaScript code starts here.

   (function($this) {
    var vendors = ['ms', 'moz', 'webkit', 'o'];
    for (var i = 0; i < vendors.length && !$this.requestAnimationFrame; ++i) {
      $this.requestAnimationFrame = $this[vendors[i] + 'RequestAnimationFrame'];
      $this.cancelAnimationFrame =
          $this[vendors[i]+'CancelAnimationFrame'] ||
          $this[vendors[i]+'CancelRequestAnimationFrame'];
    }
    if ($this.requestAnimationFrame && $this.cancelAnimationFrame) return;
    $this.requestAnimationFrame = function(callback) {
       return window.setTimeout(function() {
-        callback(DateTime.now());
+        callback(Date.now());
       }, 16 /* 16ms ~= 60fps */);
    };
    $this.cancelAnimationFrame = function(id) { clearTimeout(id); }
   })(#)""",
        this);
   }
 
   /**
    * Gets an instance of the Indexed DB factory to being using Indexed DB.
    *
@@ skipping 26 matching lines @@
   void registerPort(String name, var port) {
     var serialized = _serialize(port);
     document.documentElement.attributes['dart-port:$name'] = json.stringify(serialized);
   }
 
   @DomName('Window.console')
   Console get console => Console.safeConsole;
 
 $!MEMBERS
 }