Don't export unnamed mixins.

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

Index: pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect b/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
index 2f9029cb9c8870db7c4a28ded3d5dcd7e05de6f8..6600b708af961b4202d1061276e2e702b364a4a2 100644
--- a/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/duplicated_mixin.dart.outline.expect
@@ -7,11 +7,11 @@ class Mixin extends core::Object {
   default constructor •() → void
     ;
 }
-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
     ;
 }