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

client/html/src/DocumentFragmentWrappingImplementation.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 FilteredElementList implements ElementList {
   final Node _node;
   final NodeList _childNodes;
 
   FilteredElementList(Node node): _childNodes = node.nodes, _node = node;
 
@@ skipping 350 matching lines @@
 
   void set title(String value) {
     throw new UnsupportedOperationException(
       "Title can't be set for document fragments.");
   }
 
   void set webkitdropzone(String value) {
     throw new UnsupportedOperationException(
       "WebKit drop zone can't be set for document fragments.");
   }
+
+  DocumentFragment clone(bool deep) => super.clone(deep);
 }