[vm/nnbd] Detect null safety when running service isolate from AOT snapshot
[vm/nnbd] Detect null safety when running service isolate from AOT snapshot

AOT snapshots can be created with or without null safety, and they
include code for vm-service isolate.
When initializing vm-service isolate from AOT snapshot we should also
set isolate's null safety mode from the snapshot.

Change-Id: I2c91e1bf7fb4491144620d49e7e52be20af8b69b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/152903
Reviewed-by: Régis Crelier <c2bf536a10d9f2530c38e4e8da517718be8c39ce@google.com>
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@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