Add parser support for covariant parameters

pkg/front_end/lib/src/scanner/token.dart

 // Copyright (c) 2016, 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.
 
 /**
  * Defines the tokens that are produced by the scanner, used by the parser, and
  * referenced from the [AST structure](ast.dart).
  */
 import 'dart:collection';
 
@@ skipping 148 matching lines @@
   static const Keyword CASE = const Keyword._('CASE', "case");
 
   static const Keyword CATCH = const Keyword._('CATCH', "catch");
 
   static const Keyword CLASS = const Keyword._('CLASS', "class");
 
   static const Keyword CONST = const Keyword._('CONST', "const");
 
   static const Keyword CONTINUE = const Keyword._('CONTINUE', "continue");
 
+  static const Keyword COVARIANT =
+      const Keyword._('COVARIANT', "covariant", true);
+
   static const Keyword DEFAULT = const Keyword._('DEFAULT', "default");
 
   static const Keyword DEFERRED = const Keyword._('DEFERRED', "deferred", true);
 
   static const Keyword DO = const Keyword._('DO', "do");
 
   static const Keyword DYNAMIC = const Keyword._('DYNAMIC', "dynamic", true);
 
   static const Keyword ELSE = const Keyword._('ELSE', "else");
 
@@ skipping 71 matching lines @@
   static const List<Keyword> values = const <Keyword>[
     ABSTRACT,
     AS,
     ASSERT,
     BREAK,
     CASE,
     CATCH,
     CLASS,
     CONST,
     CONTINUE,
+    COVARIANT,
     DEFAULT,
     DEFERRED,
     DO,
     DYNAMIC,
     ELSE,
     ENUM,
     EXPORT,
     EXTENDS,
     EXTERNAL,
     FACTORY,
@@ skipping 1014 matching lines @@
  */
 class _EndOfFileTokenType extends TokenType {
   /**
    * Initialize a newly created token.
    */
   const _EndOfFileTokenType() : super._('EOF', TokenClass.NO_CLASS, '');
 
   @override
   String toString() => '-eof-';
 }