Implement toString on ModifierBuilder.

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

Index: pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart b/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
index fe46212c634eb8a0f43ccbc43b0699dfabf4a83a..114a9f680a6d125ccf3ee658ac859ec203fd16bf 100644
--- a/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
+++ b/pkg/front_end/lib/src/fasta/builder/modifier_builder.dart
@@ -42,4 +42,12 @@ abstract class ModifierBuilder extends Builder {
   String get name;
 
   bool get isNative => false;
+
+  String get debugName;
+
+  StringBuffer printOn(StringBuffer buffer) {
+    return buffer..write(name ?? fullNameForErrors);
+  }
+
+  String toString() => "$debugName(${printOn(new StringBuffer())})";
 }