[vm/ffi] Roll `package:ffi` to 0.3.1-nullsafety.0
[vm/ffi] Roll `package:ffi` to 0.3.1-nullsafety.0

Change-Id: I39efd96c98f40d0416cd738985da6f7210bfaaea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/184480
Reviewed-by: Clement Skau <495593248b6158163719b90dde8357b589c8e5d0@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