Add AstBuilder support for fields.

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

 // Copyright (c) 2016, 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.body_builder;
 
 import 'package:front_end/src/fasta/parser/parser.dart'
     show FormalParameterType, optional;
 
 import 'package:front_end/src/fasta/parser/error_kind.dart' show ErrorKind;
@@ skipping 277 matching lines @@
   @override
   void endTopLevelFields(int count, Token beginToken, Token endToken) {
     debugEvent("TopLevelFields");
     doFields(count);
     // There's no metadata here because of a slight assymetry between
     // [parseTopLevelMember] and [parseMember]. This assymetry leads to
     // DietListener discarding top-level member metadata.
   }
 
   @override
-  void endFields(int count, Token beginToken, Token endToken) {
+  void endFields(
+      int count, Token covariantKeyword, Token beginToken, Token endToken) {
     debugEvent("Fields");
     doFields(count);
     pop(); // Metadata.
   }
 
   void doFields(int count) {
     for (int i = 0; i < count; i++) {
       Expression initializer = pop();
       Identifier identifier = pop();
       if (initializer != null) {
@@ skipping 2363 matching lines @@
   } else if (node is TypeDeclarationBuilder) {
     return node.name;
   } else if (node is PrefixBuilder) {
     return node.name;
   } else if (node is ThisPropertyAccessor) {
     return node.name.name;
   } else {
     return internalError("Unhandled: ${node.runtimeType}");
   }
 }