Don't export unnamed mixins.

pkg/front_end/testcases/rasta/duplicated_mixin.dart.strong.expect

Index: pkg/front_end/testcases/rasta/duplicated_mixin.dart.strong.expect
diff --git a/pkg/front_end/testcases/rasta/duplicated_mixin.dart.direct.expect b/pkg/front_end/testcases/rasta/duplicated_mixin.dart.strong.expect
similarity index 60%
copy from pkg/front_end/testcases/rasta/duplicated_mixin.dart.direct.expect
copy to pkg/front_end/testcases/rasta/duplicated_mixin.dart.strong.expect
index 5ed76f83ff435c4a2f571498bd47498867ea403e..241032c0c0221c56917eedd0025d30b5f92a11c6 100644
--- a/pkg/front_end/testcases/rasta/duplicated_mixin.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/duplicated_mixin.dart.strong.expect
@@ -8,11 +8,11 @@ class Mixin extends core::Object {
     : super core::Object::•()
     ;
 }
-abstract class Object&Mixin = core::Object with self::Mixin {
+abstract class _Object&Mixin = core::Object with self::Mixin {
 }
-abstract class Object&Mixin&Mixin = self::Object&Mixin with self::Mixin {
+abstract class __Object&Mixin&Mixin = self::_Object&Mixin with self::Mixin {
 }
-class A extends self::Object&Mixin&Mixin {
+class A extends self::__Object&Mixin&Mixin {
   default constructor •() → void
     : super core::Object::•()
     ;