Restore old supermixin functionality to shared language tests.

tests/language/mixin_super_2_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 B {
   // 'super' resolves to Object, and in some tests, multiple points in the
   // inheritance chain.
   toString() => 'B(' + super.toString() + ')';
 }
 
 class R {
@@ skipping 19 matching lines @@
     Expect.equals(expected, object.toString());
   }
 
   check(new B(), "B(Instance of 'B')");
   check(new R(), "R[Instance of 'R']");
   check(new D(), "D<B(R[Instance of 'D'])>");
   check(new E(), "E{B(D<B(R[Instance of 'E'])>)}");
   check(new F(), "B(B(R[Instance of 'F']))");
   check(new G(), "G{B(B(B(R[Instance of 'G'])))}");
 }