Add AstBuilder support for fields.

pkg/compiler/lib/src/parser/node_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.node_listener;
 
 import '../common.dart';
 import '../elements/elements.dart' show CompilationUnitElement;
 import 'package:front_end/src/fasta/parser/parser.dart'
     show FormalParameterType;
@@ skipping 646 matching lines @@
   void endMember() {
     // TODO(sigmund): consider moving metadata into each declaration
     // element instead.
     Node node = popNode(); // member
     popNode(); // Discard metadata
     pushNode(node);
     super.endMember();
   }
 
   @override
-  void endFields(int count, Token beginToken, Token endToken) {
+  void endFields(
+      int count, Token covariantKeyword, Token beginToken, Token endToken) {
     NodeList variables = makeNodeList(count, null, endToken, ",");
     TypeAnnotation type = popNode();
     Modifiers modifiers = popNode();
     pushNode(new VariableDefinitions(type, modifiers, variables));
   }
 
   @override
   void endMethod(Token getOrSet, Token beginToken, Token endToken) {
     Statement body = popNode();
     AsyncModifier asyncModifier = popNode();
@@ skipping 334 matching lines @@
     }
     lastErrorWasNativeFunctionBody = false;
   }
 
   void internalError({Token token, Node node}) {
     // TODO(ahe): This should call reporter.internalError.
     Spannable spannable = (token == null) ? node : token;
     throw new SpannableAssertionFailure(spannable, 'Internal error in parser.');
   }
 }