Improve upper bound finalization in VM and fix #25122.

tests/language/regress_25122_test.dart

Index: tests/language/regress_25122_test.dart
diff --git a/tests/isolate/exit_at_spawnuri_iso.dart b/tests/language/regress_25122_test.dart
similarity index 58%
copy from tests/isolate/exit_at_spawnuri_iso.dart
copy to tests/language/regress_25122_test.dart
index 9bc892ee55a5886276c53ae4481ec2c1e49a9112..b7d4068feebf2768730009889c6a95b8bd9d0e5c 100644
--- a/tests/isolate/exit_at_spawnuri_iso.dart
+++ b/tests/language/regress_25122_test.dart
@@ -2,6 +2,9 @@
 // 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.
 
-library exit_at_spawn_iso;
-
 main() {}
+
+class AbstractListMember<E, M extends AbstractListMember<E, M>> {}
+
+class RepoListMember<M extends RepoListMember<M>>
+    extends AbstractListMember<String, M> {}