Implement toString on ModifierBuilder.

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

Index: pkg/front_end/lib/src/fasta/builder/void_type_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/void_type_builder.dart b/pkg/front_end/lib/src/fasta/builder/void_type_builder.dart
index 11d004d56e265405cee34308fe89d5373634098d..2ccffe53768183caac1df999b271cebb4dde871f 100644
--- a/pkg/front_end/lib/src/fasta/builder/void_type_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/void_type_builder.dart
@@ -10,4 +10,6 @@ class VoidTypeBuilder<T extends TypeBuilder, R>
     extends BuiltinTypeBuilder<T, R> {
   VoidTypeBuilder(R type, LibraryBuilder compilationUnit, int charOffset)
       : super("void", type, compilationUnit, charOffset);
+
+  String get debugName => "VoidTypeBuilder";
 }