Implement toString on ModifierBuilder.

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

Index: pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart b/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
index 27b96a0e5a72739f265026056c9faca6e81ac9de..e5875a4c551cf579d2f31aad3489efc9b5310186 100644
--- a/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/procedure_builder.dart
@@ -44,6 +44,8 @@ abstract class ProcedureBuilder<T extends TypeBuilder> extends MemberBuilder {
       int charOffset)
       : super(compilationUnit, charOffset);
 
+  String get debugName => "ProcedureBuilder";
+
   AsyncMarker get asyncModifier;
 
   ProcedureKind get kind;