Restore old supermixin functionality to shared language tests.

tests/language/mixin_superclass_test.dart

Index: tests/language/mixin_superclass_test.dart
diff --git a/tests/language/mixin_superclass_test.dart b/tests/language/mixin_superclass_test.dart
index ade41da4ec7373dae7832a1a56282310a519c817..1e658e8972432ab3576aa7ef529d597cbf5ddb91 100644
--- a/tests/language/mixin_superclass_test.dart
+++ b/tests/language/mixin_superclass_test.dart
@@ -1,6 +1,7 @@
 // Copyright (c) 2013, 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
 
 class S0 { }
 class S1 extends Object { }