Restore old supermixin functionality to shared language tests.

tests/language/mixin_superclass_test.dart

 // 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 { }
 class S2 extends S0 { }
 
 class M0 { }
 class M1 extends Object { }
 class M2 extends M0 { }
 
 class C00 = S0 with M0;
@@ skipping 98 matching lines @@
 
   new D20();
   new D21();
   new D22();
   new D23();
   new D24();
   new D25();
   new D26();
   new D27();
 }