[co19] Enable tests migrated to null-safety
[co19] Enable tests migrated to null-safety

Change-Id: I738f5be6fe60aa4addb925101eb7f2912627ec8b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/148527
Reviewed-by: Sergey Grekhov <1b71ff1db4472138d75a6e3e517692f98678cae2@unipro.ru>
Reviewed-by: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@google.com>
Commit-Queue: William Hesse <a821cddceae7dc400f272e3cb1a72f400f9fed6d@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