Add AstBuilder support for fields.

pkg/compiler/lib/src/parser/member_listener.dart

 // Copyright (c) 2015, 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 dart2js.parser.member_listener;
 
 import '../common.dart';
 import '../elements/elements.dart' show Element, ElementKind, Elements;
 import '../elements/modelx.dart'
     show ClassElementX, ElementX, FieldElementX, VariableList;
@@ skipping 97 matching lines @@
         name,
         beginToken,
         endToken,
         ElementKind.FACTORY_CONSTRUCTOR,
         method.modifiers,
         enclosingClass);
     addMember(memberElement);
   }
 
   @override
-  void endFields(int count, Token beginToken, Token endToken) {
+  void endFields(
+      int count, Token covariantKeyword, Token beginToken, Token endToken) {
     bool hasParseError = memberErrors.head;
-    super.endFields(count, beginToken, endToken);
+    super.endFields(count, covariantKeyword, beginToken, endToken);
     VariableDefinitions variableDefinitions = popNode();
     Modifiers modifiers = variableDefinitions.modifiers;
     pushNode(null);
     void buildFieldElement(Identifier name, VariableList fields) {
       Element element = new FieldElementX(name, enclosingClass, fields);
       addMember(element);
     }
 
     buildFieldElements(modifiers, variableDefinitions.definitions,
         enclosingClass, buildFieldElement, beginToken, endToken, hasParseError);
@@ skipping 19 matching lines @@
     addMetadata(memberElement);
     enclosingClass.addMember(memberElement, reporter);
   }
 
   @override
   void endMetadata(Token beginToken, Token periodBeforeName, Token endToken) {
     super.endMetadata(beginToken, periodBeforeName, endToken);
     pushMetadata(new PartialMetadataAnnotation(beginToken, endToken));
   }
 }