Use NnbdMode.Strong for frontend_server/ binary protocol.
Use NnbdMode.Strong for frontend_server/ binary protocol.

Bug: https://github.com/dart-lang/sdk/issues/50100
Change-Id: Ibbacffd90c49d1446df4a59fd718b53017dda0f0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/262441
Commit-Queue: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@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