Fix owner of mixin forwarding constructors

tests/language/regress_25246_2.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.
 
-class A {
+import 'regress_25246_3.dart';
+
+class MixIn {
+  var test3 = new Test3(() { });
+  void test() {
+    test3.test();
+  }
 }
-
-class C extends A {
-  foo() {
-    super.foo = 3;
-  }
-}