[vm] Add nnbd-experiment to the snapshot feature string
[vm] Add nnbd-experiment to the snapshot feature string

This means that when snapshots are loaded, the nnbd experiment flag they
were compiled with must match the flag the VM currently has.

Bug: https://github.com/dart-lang/sdk/issues/37479
Change-Id: I907a68f49c7c1ac4467c4c39ce033075eba457a1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/127961
Reviewed-by: Régis Crelier <c2bf536a10d9f2530c38e4e8da517718be8c39ce@google.com>
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Liam Appelbe <78c2029e58e395825ba7228e218c841bb436a9e9@google.com>

dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk