Add AstBuilder support for fields.

pkg/front_end/lib/src/fasta/parser/listener.dart

 // Copyright (c) 2012, 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 fasta.parser.listener;
 
 import '../scanner/token.dart' show BeginGroupToken, Token;
 
 import '../util/link.dart' show Link;
 
@@ skipping 161 matching lines @@
   void handleNoFormalParameters(Token token) {
     logEvent("NoFormalParameters");
   }
 
   void beginFormalParameters(Token token) {}
 
   void endFormalParameters(int count, Token beginToken, Token endToken) {
     logEvent("FormalParameters");
   }
 
+  /// Handle the end of a field declaration.  Substructures:
+  /// - Metadata
+  /// - Modifiers
+  /// - Type
+  /// - Variable declarations (count times)
+  ///
   /// Doesn't have a corresponding begin event, use [beginMember] instead.
-  void endFields(int count, Token beginToken, Token endToken) {
+  void endFields(
+      int count, Token covariantKeyword, Token beginToken, Token endToken) {
     logEvent("Fields");
   }
 
   void beginForStatement(Token token) {}
 
   void endForStatement(
       int updateExpressionCount, Token beginToken, Token endToken) {
     logEvent("ForStatement");
   }
 
@@ skipping 765 matching lines @@
 
   final Map arguments;
 
   ParserError(this.beginOffset, this.endOffset, this.kind, this.arguments);
 
   ParserError.fromTokens(Token begin, Token end, ErrorKind kind, Map arguments)
       : this(begin.charOffset, end.charOffset + end.charCount, kind, arguments);
 
   String toString() => "@${beginOffset}: $kind $arguments";
 }