Fix finalization of cyclic mixin types (issue 22666).

tests/language/regress_22666_test.dart

Index: tests/language/regress_22666_test.dart
===================================================================
--- tests/language/regress_22666_test.dart	(revision 0)
+++ tests/language/regress_22666_test.dart	(working copy)
@@ -0,0 +1,9 @@
+// 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.
+
+import 'dart:collection';
+
+class A extends Object with LinkedListEntry<A> { }
+
+main() => new A();