This is the second part of the referenced issue.
Practically, we don't do anything special during demotion, we merely
demote when write T to such type S, that T <: S. This S must be a
type of interest (tested or declared). If it happens to be nullable,
and T < NonNull(S), we will promote to NonNull(S).
Bug: https://github.com/dart-lang/sdk/issues/40011
Change-Id: I254b4bfbbfad47fa2c2811def864c8ce0419b7c9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/145740
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Konstantin Shcheglov
0662b64697