Allow for-in to use a previously declared identifier as variable.

frog/leg/warnings.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 MessageKind {
   final String template;
   const MessageKind(this.template);
 
   static final GENERIC = const MessageKind('#{1}');
 
@@ skipping 40 matching lines @@
   static final NO_THIS_IN_STATIC = const MessageKind(
       '\'this\' is only available in instance methods');
   static final DUPLICATE_INITIALIZER = const MessageKind(
       'field #{1} is initialized more than once');
   static final ALREADY_INITIALIZED = const MessageKind(
       '#{1} was already initialized here');
   static final INIT_STATIC_FIELD = const MessageKind(
       'cannot initialize static field #{1}');
   static final NOT_A_FIELD = const MessageKind(
       '#{1} is not a field');
+  static final INVALID_FOR_IN = const MessageKind(
+      'Invalid for-in variable declaration.');
 
   toString() => template;
 }
 
 class Message {
   final kind;
   final List arguments;
   String message;
 
   Message(this.kind, this.arguments);
@@ skipping 32 matching lines @@
   String toString() => message.toString();
 }
 
 class ResolutionWarning {
   final Message message;
   ResolutionWarning.message(this.message);
   ResolutionWarning(MessageKind kind, List<Type> arguments)
     : message = new Message(kind, arguments);
   String toString() => message.toString();
 }