More work on finalization of recursive types (fixes #29357).

tests/language/regress_29357_test.dart

+// Copyright (c) 2017, 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.
+
+abstract class A<T extends A<T>> {}
+
+abstract class B<T extends A<T>> {}
+
+class C<U> extends B<D<U>> {}
+
+class D<U> extends A<D<U>> {}
+
+main() {
+  new D();
+  new C();
+  new D<C>();
+  new C<D>();
+}