Properly type the clone method for Element and a few of its subclasses.

client/html/src/ElementWrappingImplementation.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.
 
 // TODO(jacobr): use Lists.dart to remove some of the duplicated functionality.
 class _ChildrenElementList implements ElementList {
   // Raw Element.
   final _element;
   final _childElements;
 
@@ skipping 746 matching lines @@
             dom.window.getComputedStyle(_ptr, pseudoElement)),
         new Completer<CSSStyleDeclaration>());
   }
 
   ElementEvents get on() {
     if (_on === null) {
       _on = new ElementEventsImplementation._wrap(_ptr);
     }
     return _on;
   }
+
+  Element clone(bool deep) => super.clone(deep);
 }