Add parser support for covariant parameters

pkg/analyzer/lib/src/error/codes.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.error.codes;
 
 import 'package:analyzer/error/error.dart';
 
 export 'package:analyzer/src/analysis_options/error/option_codes.dart';
 export 'package:analyzer/src/dart/error/hint_codes.dart';
@@ skipping 1439 matching lines @@
    * 0: the name of the type parameter
    */
   static const CompileTimeErrorCode INVALID_TYPE_ARGUMENT_IN_CONST_MAP =
       const CompileTimeErrorCode(
           'INVALID_TYPE_ARGUMENT_IN_CONST_MAP',
           "Constant map literals can't include a type parameter as a type "
           "argument, such as '{0}'.",
           "Try replacing the type parameter with a different type.");
 
   /**
+   * The 'covariant' keyword was found in an inappropriate location.
+   */
+  static const CompileTimeErrorCode INVALID_USE_OF_COVARIANT =
+      const CompileTimeErrorCode(
+          'INVALID_USE_OF_COVARIANT',
+          "The 'covariant' keyword can only be used for parameters in instance "
+          "methods or before non-final instance fields.",
+          "Try removing the 'covariant' keyword.");
+
+  /**
    * 14.2 Exports: It is a compile-time error if the compilation unit found at
    * the specified URI is not a library declaration.
    *
    * 14.1 Imports: It is a compile-time error if the compilation unit found at
    * the specified URI is not a library declaration.
    *
    * 14.3 Parts: It is a compile time error if the contents of the URI are not a
    * valid part declaration.
    *
    * Parameters:
@@ skipping 3435 matching lines @@
    * created from the optional [correction] template.
    */
   const StrongModeCode(ErrorType type, String name, String message,
       [String correction])
       : type = type,
         super('STRONG_MODE_$name', message, correction);
 
   @override
   ErrorSeverity get errorSeverity => type.severity;
 }