Implement toString on ModifierBuilder.

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

Index: pkg/front_end/lib/src/fasta/builder/field_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/field_builder.dart b/pkg/front_end/lib/src/fasta/builder/field_builder.dart
index 0d84efb4d3f234cf50d6d2fbd266cf0c9c2da8e4..7782a3be9ad82491d4cf50aba209ac0931a0247c 100644
--- a/pkg/front_end/lib/src/fasta/builder/field_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/field_builder.dart
@@ -17,6 +17,8 @@ abstract class FieldBuilder<T> extends MemberBuilder {
       this.name, this.modifiers, LibraryBuilder compilationUnit, int charOffset)
       : super(compilationUnit, charOffset);
 
+  String get debugName => "FieldBuilder";
+
   DartType get builtType;
 
   void set initializer(T value);