Fix owner of mixin forwarding constructors

tests/language/regress_25246_3.dart

Index: tests/language/regress_25246_3.dart
diff --git a/tests/language/deferred_super_dependency_lib.dart b/tests/language/regress_25246_3.dart
similarity index 76%
copy from tests/language/deferred_super_dependency_lib.dart
copy to tests/language/regress_25246_3.dart
index a592dabbae9fbb321b2f5c088e1267997af0e6fa..ce61a2eaca9bcf534e65cb8e85c6043afdce2636 100644
--- a/tests/language/deferred_super_dependency_lib.dart
+++ b/tests/language/regress_25246_3.dart
@@ -2,11 +2,10 @@
 // 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 {
-}
-
-class C extends A {
-  foo() {
-    super.foo = 3;
+class Test3 {
+  final fn;
+  Test3(this.fn);
+  void test() {
+    fn();
   }
-}
+}