Don't export unnamed mixins.

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

Index: pkg/front_end/testcases/rasta/issue_000007.dart.strong.expect
diff --git a/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect b/pkg/front_end/testcases/rasta/issue_000007.dart.strong.expect
similarity index 76%
copy from pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
copy to pkg/front_end/testcases/rasta/issue_000007.dart.strong.expect
index 39373ca5cbc21da7df80d24931950b1d13f5ef71..1947a7d36e66c5a7a850f26b3f5ebcf75f55ae17 100644
--- a/pkg/front_end/testcases/rasta/issue_000007.dart.direct.expect
+++ b/pkg/front_end/testcases/rasta/issue_000007.dart.strong.expect
@@ -14,13 +14,13 @@ 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::•()
     ;
 }
 static method main() → dynamic {
-  new self::Sub::•().foo();
+  new self::Sub::•().{self::Mixin::foo}();
 }