Disallow mixing in classes where the superclass != Object.

tests/language/mixin_mixin_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.
 
+class M1 { foo() => 42; }
+typedef M2 = Object with M1;
+
 class S { }
-class M1 { foo() => 42; }
-typedef M2 = S with M1;
 typedef C = S with M2;
 
 main() {
   var c = new C();
   Expect.isTrue(c is S);
   Expect.isTrue(c is M1);
   Expect.isTrue(c is M2);
   Expect.isTrue(c is C);
   Expect.equals(42, c.foo());
 }