Add warnings and hints to for-in.

tests/compiler/dart2js/mock_compiler.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.
 
 library mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:async';
 import 'dart:collection';
 
@@ skipping 352 matching lines @@
   if (expectedIterator.hasNext) {
     do {
       var expected = expectedIterator.next();
       if (expected is CheckMessage) expected = expected(null);
       print('Expected $kind "${expected}" did not occur');
     } while (expectedIterator.hasNext);
     fail('Too few ${kind}s');
   }
   if (foundIterator.hasNext) {
     do {
-      print('Additional $kind "${foundIterator.next()}"');
+      WarningMessage message = foundIterator.next();
+      print('Additional $kind "${message}: ${message.message}"');
     } while (foundIterator.hasNext);
     fail('Too many ${kind}s');
   }
 }
 
 class CollectingTreeElements extends TreeElementMapping {
   final Map<Node, Element> map = new LinkedHashMap<Node, Element>();
 
   CollectingTreeElements(Element currentElement) : super(currentElement);
 
@@ skipping 45 matching lines @@
   MockElement(Element enclosingElement)
       : super('', ElementKind.FUNCTION, Modifiers.EMPTY,
               enclosingElement);
 
   get node => null;
 
   parseNode(_) => null;
 
   bool get hasNode => false;
 }