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

This reverts commit 61b4bbcb6b45b5d4eea2d81885f88478456f6f10.

Reason for revert: Performance regressions

Original change's description:
> [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 <asiva@google.com>
> Commit-Queue: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@google.com>

TBR=asiva@google.com,78c2029e58e395825ba7228e218c841bb436a9e9@google.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: https://github.com/dart-lang/sdk/issues/40179
Change-Id: I7e1d81286d769faeacec7dc17612ff18d157f114
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/134060
Reviewed-by: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@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