Implement toString on ModifierBuilder.

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

Index: pkg/front_end/lib/src/fasta/builder/invalid_type_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/invalid_type_builder.dart b/pkg/front_end/lib/src/fasta/builder/invalid_type_builder.dart
index 5b646010d4631fdbafe606ed906501c55388622f..945f7e140bb640e99232a7180383ac1318b91404 100644
--- a/pkg/front_end/lib/src/fasta/builder/invalid_type_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/invalid_type_builder.dart
@@ -10,4 +10,6 @@ abstract class InvalidTypeBuilder<T extends TypeBuilder, R>
     extends TypeDeclarationBuilder<T, R> {
   InvalidTypeBuilder(String name, int charOffset, [Uri fileUri])
       : super(null, 0, name, null, charOffset, fileUri);
+
+  String get debugName => "InvalidTypeBuilder";
 }