Implement toString on ModifierBuilder.

pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart

Index: pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart b/pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart
index 688738807d9d9de6a8897c1c4a59a182bdc8756d..131c282649cd7081055848b08fc969c96903af00 100644
--- a/pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart
+++ b/pkg/front_end/lib/src/fasta/dill/dill_member_builder.dart
@@ -28,6 +28,8 @@ class DillMemberBuilder extends MemberBuilder {
         member = member,
         super(parent, member.fileOffset);
 
+  String get debugName => "DillMemberBuilder";
+
   Member get target => member;
 
   String get name => member.name.name;