[cfe] Enable NNBD support in VM for NNBD unit tests
[cfe] Enable NNBD support in VM for NNBD unit tests

Change-Id: I8ab19dcf5e5b94b2192f13ecbce917dfa8c0fb40
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/138281
Commit-Queue: Dmitry Stefantsov <95a4b7e39916a54bd6477bec6a6de014b977db32@google.com>
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@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