Fix Enum.toString when enum type is private.

tests/language/enum_test.dart

Index: tests/language/enum_test.dart
diff --git a/tests/language/enum_test.dart b/tests/language/enum_test.dart
index 84daba1143b034f0ad499ecf0727e9c5e1e19da8..09395eb7b3213e0d30cf2bfce9492bd028ed8940 100644
--- a/tests/language/enum_test.dart
+++ b/tests/language/enum_test.dart
@@ -11,6 +11,7 @@ enum Enum2 { A }
 enum Enum3 { B, C }
 enum Enum4 { D, E, }
 enum Enum5 { F, G, H }
+enum _Enum6 { I, _J }
 
 main() {
   Expect.equals('Enum1._', Enum1._.toString());
@@ -45,6 +46,9 @@ main() {
   Expect.equals(2, Enum5.H.index);
   Expect.listEquals([Enum5.F, Enum5.G, Enum5.H], Enum5.values);
   Enum5.values.forEach(test5);
+
+  Expect.equals('_Enum6.I', _Enum6.I.toString());
+  Expect.equals('_Enum6._J', _Enum6._J.toString());
 }
 
 test1(Enum1 e) {