Support 'Conditional' idl attribute.

client/dom/generated/src/interface/MouseEvent.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.
 
 // WARNING: Do not edit - generated code.
 
 interface MouseEvent extends UIEvent {
 
   bool get altKey();
 
@@ skipping 18 matching lines @@
   EventTarget get relatedTarget();
 
   int get screenX();
 
   int get screenY();
 
   bool get shiftKey();
 
   Node get toElement();
 
-  int get webkitMovementX();
-
-  int get webkitMovementY();
-
   int get x();
 
   int get y();
 
   void initMouseEvent(String type, bool canBubble, bool cancelable, DOMWindow view, int detail, int screenX, int screenY, int clientX, int clientY, bool ctrlKey, bool altKey, bool shiftKey, bool metaKey, int button, EventTarget relatedTarget);
 }