Restore old supermixin functionality to shared language tests.

tests/language/mixin_super_test.dart

Index: tests/language/mixin_super_test.dart
diff --git a/tests/language/mixin_super_test.dart b/tests/language/mixin_super_test.dart
index 6a5d5a378cce82f45515929da410351117edaefd..8a4ee98eb73ba195323f46e98f492d334aaeeb8d 100644
--- a/tests/language/mixin_super_test.dart
+++ b/tests/language/mixin_super_test.dart
@@ -1,6 +1,7 @@
 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
+// SharedOptions=--supermixin
 
 import "package:expect/expect.dart";