Add AstBuilder support for fields.

pkg/front_end/lib/src/fasta/source/outline_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.outline_builder;
 
 import 'package:kernel/ast.dart' show AsyncMarker, ProcedureKind;
 
 import '../parser/parser.dart' show FormalParameterType, optional;
 
@@ skipping 473 matching lines @@
     debugEvent("endTopLevelFields");
     List<String> names = popList(count);
     TypeBuilder type = pop();
     int modifiers = Modifier.validate(pop());
     List<MetadataBuilder> metadata = pop();
     library.addFields(metadata, modifiers, type, names);
     checkEmpty(beginToken.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);
     TypeBuilder type = pop();
     int modifiers = Modifier.validate(pop());
     List<MetadataBuilder> metadata = pop();
     library.addFields(metadata, modifiers, type, names);
   }
 
   @override
   void endTypeVariable(Token token, Token extendsOrSuper) {
@@ skipping 118 matching lines @@
   Link<Token> handleMemberName(Link<Token> identifiers) {
     if (!isDartLibrary || identifiers.isEmpty) return identifiers;
     return removeNativeClause(identifiers);
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }