Don't export unnamed mixins.

pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect

Index: pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect b/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
index d7371d50823cbe58c162f795b18fd8422439ad3f..4ab4b9010809bfa9457e94f96133f7921fc699c1 100644
--- a/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
+++ b/pkg/front_end/testcases/rasta/issue_000007.dart.outline.expect
@@ -12,9 +12,9 @@ class Mixin extends core::Object {
   method foo() → dynamic
     ;
 }
-abstract class Base&Mixin = self::Base with self::Mixin {
+abstract class _Base&Mixin = self::Base with self::Mixin {
 }
-class Sub extends self::Base&Mixin {
+class Sub extends self::_Base&Mixin {
   default constructor •() → void
     ;
 }