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

client/html/src/Element.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 ElementList extends List<Element> {
   // TODO(jacobr): add element batch manipulation methods.
   Element get first();
   // TODO(jacobr): add insertAt
 }
 
@@ skipping 174 matching lines @@
 
   bool matchesSelector([String selectors]);
 
   Future<ElementRect> get rect();
 
   Future<CSSStyleDeclaration> get computedStyle();
 
   Future<CSSStyleDeclaration> getComputedStyle(String pseudoElement);
 
   ElementEvents get on();
+
+  Element clone(bool deep);
 }