Add some sweet XML classes.

client/html/src/XMLHttpRequest.dart

 // Copyright (c) 2011, 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.
 
 interface XMLHttpRequestEvents extends Events {
   EventListenerList get abort();
   EventListenerList get error();
   EventListenerList get load();
   EventListenerList get loadStart();
   EventListenerList get progress();
@@ skipping 18 matching lines @@
   XMLHttpRequest.getTEMPNAME(String url, onSuccess(XMLHttpRequest request));
 
   int get readyState();
 
   String get responseText();
 
   String get responseType();
 
   void set responseType(String value);
 
-  Document get responseXML();
+  XMLDocument get responseXML();
 
   int get status();
 
   String get statusText();
 
   XMLHttpRequestUpload get upload();
 
   bool get withCredentials();
 
   void set withCredentials(bool value);
 
   void abort();
 
   String getAllResponseHeaders();
 
   String getResponseHeader(String header);
 
   void open(String method, String url, bool async, [String user, String password]);
 
   void overrideMimeType(String mime);
 
   void send([String data]);
 
   void setRequestHeader(String header, String value);
 
   XMLHttpRequestEvents get on();
 }