Implement toString on ModifierBuilder.

pkg/front_end/lib/src/fasta/dill/dill_member_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.dill_member_builder;
 
 import 'package:kernel/ast.dart'
     show Constructor, Field, Member, Procedure, ProcedureKind;
 
 import '../errors.dart' show internalError;
@@ skipping 10 matching lines @@
 class DillMemberBuilder extends MemberBuilder {
   final int modifiers;
 
   final Member member;
 
   DillMemberBuilder(Member member, Builder parent)
       : modifiers = computeModifiers(member),
         member = member,
         super(parent, member.fileOffset);
 
+  String get debugName => "DillMemberBuilder";
+
   Member get target => member;
 
   String get name => member.name.name;
 
   bool get isConstructor => member is Constructor;
 
   ProcedureKind get kind {
     final member = this.member;
     return member is Procedure ? member.kind : null;
   }
@@ skipping 31 matching lines @@
   } else if (member is Procedure) {
     modifier |= member.isConst ? constMask : 0;
     modifier |= member.isStatic ? staticMask : 0;
   } else if (member is Constructor) {
     modifier |= member.isConst ? constMask : 0;
   } else {
     internalError("Unhandled: ${member.runtimeType}");
   }
   return modifier;
 }