Restore old supermixin functionality to shared language tests.

tests/language/mixin_super_use_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
 
 import "package:expect/expect.dart";
 
 class M {
 }
 
 class P0 {
   foo() {
     super.toString();
 
@@ skipping 42 matching lines @@
 class E = Object with M, P1;
 class F = Object with P2, M;
 
 main() {
   var p1 = new P1();
   var p2 = new P2();
   Expect.equals(87, p1.bar());
   p1.test();
   Expect.equals(99, p2.baz());
 }