Add AstBuilder support for fields.

pkg/front_end/lib/src/fasta/source/diet_listener.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.diet_listener;
 
 import 'package:kernel/ast.dart' show AsyncMarker;
 
 import 'package:kernel/class_hierarchy.dart' show ClassHierarchy;
 
@@ skipping 160 matching lines @@
 
   @override
   void endFunctionTypeAlias(
       Token typedefKeyword, Token equals, Token endToken) {
     debugEvent("FunctionTypeAlias");
     discard(2); // Name + endToken.
     checkEmpty(typedefKeyword.charOffset);
   }
 
   @override
-  void endFields(int count, Token beginToken, Token endToken) {
+  void endFields(
+      int count, Token covariantToken, Token beginToken, Token endToken) {
     debugEvent("Fields");
     List<String> names = popList(count);
     Builder builder = lookupBuilder(beginToken, null, names.first);
     buildFields(beginToken, false, builder);
   }
 
   @override
   void handleAsyncModifier(Token asyncToken, Token startToken) {
     debugEvent("AsyncModifier");
   }
@@ skipping 361 matching lines @@
   bool get isTargetingDartVm {
     // TODO(ahe): Find a more reliable way to check if this is the Dart VM.
     return !coreTypes.containsLibrary("dart:_js_helper");
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }