Stop creating duplicate enum members when the element model has already been built

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.
 
 library analyzer.dart.ast.ast;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/src/dart/ast/ast.dart';
 import 'package:analyzer/src/dart/element/element.dart' show AuxiliaryElements;
@@ skipping 2838 matching lines @@
       SimpleIdentifier name,
       Token leftBracket,
       List<EnumConstantDeclaration> constants,
       Token rightBracket) = EnumDeclarationImpl;
 
   /**
    * Return the enumeration constants being declared.
    */
   NodeList<EnumConstantDeclaration> get constants;
 
+  @override
+  ClassElement get element;
+
   /**
    * Return the 'enum' keyword.
    */
   Token get enumKeyword;
 
   /**
    * Set the 'enum' keyword to the given [token].
    */
   void set enumKeyword(Token token);
 
@@ skipping 5107 matching lines @@
   /**
    * Return the 'yield' keyword.
    */
   Token get yieldKeyword;
 
   /**
    * Return the 'yield' keyword to the given [token].
    */
   void set yieldKeyword(Token token);
 }