Implement toString on ModifierBuilder.

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

Index: pkg/front_end/lib/src/fasta/builder/class_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/class_builder.dart b/pkg/front_end/lib/src/fasta/builder/class_builder.dart
index 33f0d176fc93ca2e4b4c220599303c288c9e3207..9f462c8a32138a67a45472f03741185914d31ab2 100644
--- a/pkg/front_end/lib/src/fasta/builder/class_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/class_builder.dart
@@ -52,6 +52,8 @@ abstract class ClassBuilder<T extends TypeBuilder, R>
         constructorScopeBuilder = new ScopeBuilder(constructors),
         super(metadata, modifiers, name, parent, charOffset);
 
+  String get debugName => "ClassBuilder";
+
   /// Returns true if this class is the result of applying a mixin to its
   /// superclass.
   bool get isMixinApplication => mixedInType != null;