Implement toString on ModifierBuilder.

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

Index: pkg/front_end/lib/src/fasta/builder/builtin_type_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/builtin_type_builder.dart b/pkg/front_end/lib/src/fasta/builder/builtin_type_builder.dart
index ce8ecc170d8226e50bc96fa48dfa12c24b7f9b34..40e9cdffc4c3c3dd2de6e83d4c777fbdc90def34 100644
--- a/pkg/front_end/lib/src/fasta/builder/builtin_type_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/builtin_type_builder.dart
@@ -19,4 +19,6 @@ class BuiltinTypeBuilder<T extends TypeBuilder, R>
   R buildTypesWithBuiltArguments(LibraryBuilder library, List<R> arguments) {
     return type;
   }
+
+  String get debugName => "BuiltinTypeBuilder";
 }