Restore old supermixin functionality to shared language tests.

tests/language/mixin_super_test.dart

 // 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";
 
 class MS<T> {
   foo() {
     return "MS<$T>.foo\n";
   }
 }
 
 class M<T> extends MS<List<T>> {
@@ skipping 57 matching lines @@
                 "N<bool>.foo\n"
                 "MNA2<int, String, bool>.foo\n",
                 new MNA2<int, String, bool>().foo());
   Expect.equals("S<List<int>>.foo\n"
                 "M<Map<int, String>>.foo\n"
                 "N<bool>.foo\n"
                 "MNA3<int, String, bool>.foo\n",
                 new MNA3<int, String, bool>().foo());
 }