This should make the Link implementation work in strong mode.
We may find situations where Link<Null> leaks out, but I believe those should
be fixed by passing explicitly typed tail arguments.
Change-Id: I4c48769569f22e987ace586f5ec9264d75b49982
Reviewed-on: https://dart-review.googlesource.com/56663
Commit-Queue: Peter von der Ahé <ahe@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>