[VM] Auto-detection of null-safety mode doesn't work for AOT snapshots

[alexmarkov]

$ pkg/vm/tool/precompiler2 --enable-experiment=non-nullable --null-safety foo.dart foo.snapshot

$ pkg/vm/tool/dart_precompiled_runtime2 foo.snapshot
VM initialization failed: Snapshot not compatible with the current VM configuration: the snapshot requires 'release no-dwarf_stack_traces_mode causal_async_stacks no-lazy_async_stacks use_bare_instructions no-dedup_instructions no-"asserts" no-code-comments x64-sysv null-safety' but the VM has 'release no-dwarf_stack_traces_mode causal_async_stacks no-lazy_async_stacks use_bare_instructions no-dedup_instructions no-"asserts" no-code-comments x64-sysv no-null-safety'

$ pkg/vm/tool/dart_precompiled_runtime2 --null-safety foo.snapshot
Hello, Dart!

/cc @a-siva

[a-siva]

https://dart-review.googlesource.com/c/sdk/+/151866