Add some sweet XML classes.

client/html/src/XMLHttpRequestWrappingImplementation.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.
 
 class XMLHttpRequestEventsImplementation extends EventsImplementation
     implements XMLHttpRequestEvents {
   XMLHttpRequestEventsImplementation._wrap(_ptr) : super._wrap(_ptr);
 
   EventListenerList get abort() => _get('abort');
   EventListenerList get error() => _get('error');
@@ skipping 35 matching lines @@
   }
 
   int get readyState() => _ptr.readyState;
 
   String get responseText() => _ptr.responseText;
 
   String get responseType() => _ptr.responseType;
 
   void set responseType(String value) { _ptr.responseType = value; }
 
-  Document get responseXML() => LevelDom.wrapDocument(_ptr.responseXML);
+  XMLDocument get responseXML() => LevelDom.wrapDocument(_ptr.responseXML);
 
   int get status() => _ptr.status;
 
   String get statusText() => _ptr.statusText;
 
   XMLHttpRequestUpload get upload() => LevelDom.wrapXMLHttpRequestUpload(_ptr.upload);
 
   bool get withCredentials() => _ptr.withCredentials;
 
   void set withCredentials(bool value) { _ptr.withCredentials = value; }
@@ skipping 55 matching lines @@
     _ptr.setRequestHeader(header, value);
   }
 
   XMLHttpRequestEvents get on() {
     if (_on === null) {
       _on = new XMLHttpRequestEventsImplementation._wrap(_ptr);
     }
     return _on;
   }
 }