Add missing AST support for covariant keyword and fix bug from previous CL

pkg/analyzer/lib/dart/ast/ast.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.
 
 /**
  * Defines the AST model. The AST (Abstract Syntax Tree) model describes the
  * syntactic (as opposed to semantic) structure of Dart code. The semantic
  * structure of the code is modeled by the
  * [element model](../element/element.dart).
  *
@@ skipping 2742 matching lines @@
 /**
  * The declaration of one or more fields of the same type.
  *
  *    fieldDeclaration ::=
  *        'static'? [VariableDeclarationList] ';'
  *
  * Clients may not extend, implement or mix-in this class.
  */
 abstract class FieldDeclaration extends ClassMember {
   /**
+   * The 'covariant' keyword, or `null` if the keyword was not used.
+   */
+  Token get covariantKeyword;
+
+  /**
    * Return the fields being declared.
    */
   VariableDeclarationList get fields;
 
   /**
    * Set the fields being declared to the given list of [fields].
    */
   void set fields(VariableDeclarationList fields);
 
   /**
@@ skipping 4256 matching lines @@
   /**
    * Return the 'yield' keyword.
    */
   Token get yieldKeyword;
 
   /**
    * Return the 'yield' keyword to the given [token].
    */
   void set yieldKeyword(Token token);
 }