Before this change we allowed to serialize some references as null,
and would only complain when trying to read the dill back in.
This CL changes that so it complains up front.
It is unknown if any of this would ever occur in practise.
Change-Id: Id5e1af89abfb88a2d4249bd439b53b062d7aeffa
Reviewed-on: https://dart-review.googlesource.com/c/85392
Reviewed-by: Kevin Millikin <kmillikin@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
Jens Johansen
6c909a0efa