Return null from stringValue and call slowToString() instead of toString().

dart/frog/leg/scanner/keyword.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.
 
 /**
  * A keyword in the Dart programming language.
  */
 class Keyword implements SourceString {
   static final Keyword BREAK = const Keyword("break");
   static final Keyword CASE = const Keyword("case");
@@ skipping 108 matching lines @@
     Map<String, Keyword> result = new LinkedHashMap<String, Keyword>();
     for (Keyword keyword in values) {
       result[keyword.syntax] = keyword;
     }
     return result;
   }
 
   int hashCode() => syntax.hashCode();
 
   bool operator ==(other) {
-    return other is SourceString && toString() == other.toString();
+    return other is SourceString && toString() == other.slowToString();
   }
 
   Iterator<int> iterator() => new StringCodeIterator(syntax);
 
   void printOn(StringBuffer sb) {
     sb.add(syntax);
   }
 
   String toString() => syntax;
+  String slowToString() => syntax;
   String get stringValue() => syntax;
 
   bool isEmtpy() => false;
 }
 
 /**
  * Abstract state in a state machine for scanning keywords.
  */
 class KeywordState {
   abstract bool isLeaf();
@@ skipping 95 matching lines @@
   final Keyword keyword;
 
   LeafKeywordState(String syntax) : keyword = Keyword.keywords[syntax];
 
   bool isLeaf() => true;
 
   KeywordState next(int c) => null;
 
   String toString() => keyword.syntax;
 }