This is needed to serialize and deserialize problems to dill and
re-issuing problems as needed.
Change-Id: Id22d1c86f43b1b08c5e5a4d133869075e9b0c100
Reviewed-on: https://dart-review.googlesource.com/c/91743
Commit-Queue: Jens Johansen <jensj@google.com>
Reviewed-by: Peter von der Ahé <ahe@google.com>