Implement 'covariant' modifier for dart2js.

pkg/compiler/lib/src/tokens/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.
 
 library dart2js.tokens.keywords;
 
 import '../util/characters.dart' as Characters show $a;
 import 'precedence.dart' show PrecedenceInfo;
 import 'precedence_constants.dart' as Precedence
     show AS_INFO, IS_INFO, KEYWORD_INFO;
@@ skipping 35 matching lines @@
     const Keyword("void"),
     const Keyword("while"),
     const Keyword("with"),
 
     // TODO(ahe): Don't think this is a reserved word.
     // See: http://dartbug.com/5579
     const Keyword("is", info: Precedence.IS_INFO),
 
     const Keyword("abstract", isBuiltIn: true),
     const Keyword("as", info: Precedence.AS_INFO, isBuiltIn: true),
+    const Keyword("covariant", isBuiltIn: true),
     const Keyword("dynamic", isBuiltIn: true),
     const Keyword("export", isBuiltIn: true),
     const Keyword("external", isBuiltIn: true),
     const Keyword("factory", isBuiltIn: true),
     const Keyword("get", isBuiltIn: true),
     const Keyword("implements", isBuiltIn: true),
     const Keyword("import", isBuiltIn: true),
     const Keyword("library", isBuiltIn: true),
     const Keyword("operator", isBuiltIn: true),
     const Keyword("part", isBuiltIn: true),
     const Keyword("set", isBuiltIn: true),
     const Keyword("static", isBuiltIn: true),
     const Keyword("typedef", isBuiltIn: true),
 
     const Keyword("hide", isPseudo: true),
     const Keyword("native", isPseudo: true),
     const Keyword("of", isPseudo: true),
     const Keyword("on", isPseudo: true),
     const Keyword("show", isPseudo: true),
     const Keyword("source", isPseudo: true),
     const Keyword("deferred", isPseudo: true),
     const Keyword("async", isPseudo: true),
     const Keyword("sync", isPseudo: true),
     const Keyword("await", isPseudo: true),
-    const Keyword("yield", isPseudo: true)
+    const Keyword("yield", isPseudo: true),
   ];
 
   final String syntax;
   final bool isPseudo;
   final bool isBuiltIn;
   final PrecedenceInfo info;
 
   static Map<String, Keyword> _keywords;
   static Map<String, Keyword> get keywords {
     if (_keywords == null) {
@@ skipping 114 matching lines @@
 /**
  * A state that has no outgoing transitions.
  */
 class LeafKeywordState extends KeywordState {
   LeafKeywordState(String syntax) : super(Keyword.keywords[syntax]);
 
   KeywordState next(int c) => null;
 
   String toString() => keyword.syntax;
 }