Fix some bugs related to the covariant keyword

pkg/analyzer/lib/src/generated/parser.dart

 // Copyright (c) 2014, 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 analyzer.src.generated.parser;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 3830 matching lines @@
         }
       } else if (keyword == Keyword.CONST) {
         if (modifiers.constKeyword != null) {
           _reportErrorForCurrentToken(
               ParserErrorCode.DUPLICATED_MODIFIER, [_currentToken.lexeme]);
           _advance();
         } else {
           modifiers.constKeyword = getAndAdvance();
         }
       } else if (keyword == Keyword.COVARIANT) {
-        if (modifiers.constKeyword != null) {
+        if (modifiers.covariantKeyword != null) {
           _reportErrorForCurrentToken(
               ParserErrorCode.DUPLICATED_MODIFIER, [_currentToken.lexeme]);
           _advance();
         } else {
           modifiers.covariantKeyword = getAndAdvance();
         }
       } else if (keyword == Keyword.EXTERNAL) {
         if (modifiers.externalKeyword != null) {
           _reportErrorForCurrentToken(
               ParserErrorCode.DUPLICATED_MODIFIER, [_currentToken.lexeme]);
@@ skipping 283 matching lines @@
     if (_matchesKeyword(Keyword.COVARIANT)) {
       // Check to ensure that 'covariant' isn't being used as the parameter name.
       Token next = _peek();
       if (_tokenMatchesKeyword(next, Keyword.FINAL) ||
           _tokenMatchesKeyword(next, Keyword.CONST) ||
           _tokenMatchesKeyword(next, Keyword.VAR) ||
           _tokenMatchesKeyword(next, Keyword.THIS) ||
           _tokenMatchesIdentifier(next)) {
         covariantKeyword = getAndAdvance();
       }
+      while (_matchesKeyword(Keyword.COVARIANT)) {
+        _reportErrorForCurrentToken(
+            ParserErrorCode.DUPLICATED_MODIFIER, [_currentToken.lexeme]);
+        _advance();
+      }
     }
     FinalConstVarOrType holder = parseFinalConstVarOrType(!inFunctionType,
         inFunctionType: inFunctionType);
     Token thisKeyword = null;
     Token period = null;
     if (_matchesKeyword(Keyword.THIS)) {
       thisKeyword = getAndAdvance();
       period = _expect(TokenType.PERIOD);
     }
     if (!_matchesIdentifier() && inFunctionType) {
@@ skipping 4340 matching lines @@
    */
   Parser_SyntheticKeywordToken(Keyword keyword, int offset)
       : super(keyword, offset);
 
   @override
   int get length => 0;
 
   @override
   Token copy() => new Parser_SyntheticKeywordToken(keyword, offset);
 }