Implement toString on ModifierBuilder.

pkg/front_end/lib/src/fasta/builder/formal_parameter_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.formal_parameter_builder;
 
 import '../parser/parser.dart' show FormalParameterType;
 
 import 'builder.dart'
     show LibraryBuilder, MetadataBuilder, ModifierBuilder, TypeBuilder;
@@ skipping 13 matching lines @@
 
   /// True if this parameter is on the form `this.name`.
   final bool hasThis;
 
   FormalParameterType kind = FormalParameterType.REQUIRED;
 
   FormalParameterBuilder(this.metadata, this.modifiers, this.type, this.name,
       this.hasThis, LibraryBuilder compilationUnit, this.charOffset)
       : super(compilationUnit, charOffset);
 
+  String get debugName => "FormalParameterBuilder";
+
   bool get isRequired => kind.isRequired;
 
   bool get isPositional => kind.isPositional || kind.isRequired;
 
   bool get isNamed => kind.isNamed;
 
   bool get isOptional => !isRequired;
 
   bool get isLocal => true;
 
   @override
   String get fullNameForErrors => name;
 
   FormalParameterBuilder forFormalParameterInitializerScope();
 }