[dartfuzz] Opt out of NNBD.
[dartfuzz] Opt out of NNBD.

Currently all programs generated by Dart fuzz have NNBD errors.

Change-Id: I6f55447ad4c2986ae4ffceff8c88ba263d40e280
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193081
Reviewed-by: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@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