Added TODO.

lib/compiler/implementation/dart_backend/utils.dart

 // Copyright (c) 2012, 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 CloningVisitor implements Visitor<Node> {
   final TreeElements originalTreeElements;
-  final TreeElements cloneTreeElements;
+  final TreeElementMapping cloneTreeElements;
 
   CloningVisitor(this.originalTreeElements)
       : cloneTreeElements = new TreeElementMapping();
 
   visit(Node node) {
     if (node === null) return null;
     final clone = node.accept(this);
     final originalElement = originalTreeElements[node];
     if (originalElement !== null) {
       cloneTreeElements[clone] = originalElement;
@@ skipping 183 matching lines @@
   visitTypeVariable(TypeVariable node) => new TypeVariable(
       visit(node.name), visit(node.bound));
 
   visitVariableDefinitions(VariableDefinitions node) => new VariableDefinitions(
       visit(node.type), visit(node.modifiers), visit(node.definitions),
       node.endToken);
 
   visitWhile(While node) => new While(
       visit(node.condition), visit(node.body), node.whileKeyword);
 }