[vm] Set non_nullable_flag() to true
[vm] Set non_nullable_flag() to true

We'll leave the deletion and cleanup of this function until later, so
that this CL is as easy to rollback as possible.

Bug: https://github.com/dart-lang/sdk/issues/40179
Change-Id: Ie583aaabd117ae06ea27a539c3bc904240a0e6bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/133429
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk