Implement toString on ModifierBuilder.

pkg/front_end/lib/src/fasta/builder/formal_parameter_builder.dart

Index: pkg/front_end/lib/src/fasta/builder/formal_parameter_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/formal_parameter_builder.dart b/pkg/front_end/lib/src/fasta/builder/formal_parameter_builder.dart
index 0a322455c14225696cd8a17f1f6c472cbdf96d53..ef3f2bf61d31fe8f3951dfd25046f4205889d45e 100644
--- a/pkg/front_end/lib/src/fasta/builder/formal_parameter_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/formal_parameter_builder.dart
@@ -31,6 +31,8 @@ abstract class FormalParameterBuilder<T extends TypeBuilder>
       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;