Fix warnings in dart2js.

dart/lib/compiler/implementation/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 const List<Keyword> values = const <Keyword> [
       const Keyword("break"),
@@ skipping 101 matching lines @@
     return this;
   }
 
   bool isEmpty() => false;
   bool isPrivate() => false;
 }
 
 /**
  * Abstract state in a state machine for scanning keywords.
  */
-class KeywordState {
+abstract class KeywordState {
   abstract bool isLeaf();
   abstract KeywordState next(int c);
   abstract Keyword get keyword;
 
   static KeywordState _KEYWORD_STATE;
   static KeywordState get KEYWORD_STATE {
     if (_KEYWORD_STATE === null) {
       List<String> strings = new List<String>(Keyword.values.length);
       for (int i = 0; i < Keyword.values.length; i++) {
         strings[i] = Keyword.values[i].syntax;
@@ skipping 86 matching lines @@
   final Keyword keyword;
 
   LeafKeywordState(String syntax) : keyword = Keyword.keywords[syntax];
 
   bool isLeaf() => true;
 
   KeywordState next(int c) => null;
 
   String toString() => keyword.syntax;
 }