Don't export unnamed mixins.

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

Index: pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect b/pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect
index e9fe72d834eb9cf7d9d2a75b3b9f1b13bc6730d1..db8820c3f4247cf5c35b7c70fe69642e171f3b6d 100644
--- a/pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/issue_000080.dart.outline.expect
@@ -9,9 +9,9 @@ class Mixin extends core::Object {
   method foo() → dynamic
     ;
 }
-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
     ;
   method foo() → dynamic