TBR fix for an unresolved reference in the dartc build

frog/server/dart_json.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.
 
 #library("json");
 
 // TODO(devoncarew): this file is a copy of client/json/dart_json.dart and should be deleted as
 // soon as it is no longer referenced from frog_server.dart.
 
 // Pure Dart implementation of JSON protocol.
@@ skipping 498 matching lines @@
     final int length = s.length;
     bool needsEscape = false;
     final charCodes = new List<int>();
     for (int i = 0; i < length; i++) {
       int charCode = s.charCodeAt(i);
       if (_needsEscape(charCode)) {
         charCodes.add(JsonTokenizer.BACKSLASH);
         needsEscape = true;
 
         if (JsonTokenizer.NEW_LINE == charCode) {
-          charCode = N_SMALL;
+          charCode = JsonTokenizer.N_SMALL;
         } else if (JsonTokenizer.LINE_FEED == charCode) {
-          charCode = R_SMALL;
+          charCode = JsonTokenizer.R_SMALL;
         }
       }
       charCodes.add(charCode);
     }
     sb.add(needsEscape ? new String.fromCharCodes(charCodes) : s);
   }
 
   void _checkCycle(final object) {
     // TODO: use Iterables.
     for (int i = 0; i < _seen.length; i++) {
@@ skipping 63 matching lines @@
         });
         _sb.add('}');
         _seen.removeLast();
         return;
 
       default:
         throw const JsonUnsupportedObjectType();
     }
   }
 }