Don't export unnamed mixins.

pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect

Index: pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect b/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
index 85e3e6af181fc4caad2c36c39c1fc0d0fba7eb4f..c0b7d4ab925c3d4ba41c5cd9abba106d35bd3453 100644
--- a/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/issue_000080.dart.direct.expect
@@ -10,9 +10,9 @@ class Mixin extends core::Object {
   method foo() → dynamic
     return 87;
 }
-abstract class Object&Mixin = core::Object with self::Mixin {
+abstract class _Object&Mixin = core::Object with self::Mixin {
 }
-class Foo extends self::Object&Mixin {
+class Foo extends self::_Object&Mixin {
   default constructor •() → void
     : super core::Object::•()
     ;