Make :: final.
Make :: final.

`List` being `sealed` does not prevent its subclasses to be
extended, so `::` must be explicitly made `final`.

scala/scala,martijnhoekstra/scala,scala/scala,lrytz/scala,martijnhoekstra/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,lrytz/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,scala/scala,scala/scala,martijnhoekstra/scala,martijnhoekstra/scala,lrytz/scala,lrytz/scala