Don't export unnamed mixins.

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

Index: pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect b/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
index 39373ca5cbc21da7df80d24931950b1d13f5ef71..9da0472b2bc9cea98941ad04707c81ff74d03262 100644
--- a/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
@@ -14,9 +14,9 @@ class Mixin extends core::Object {
   method foo() → dynamic
     return core::print("foo");
 }
-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
     : super self::Base::•()
     ;