[ffi] Fix incorrect kernel type for size in FFI transform.
[ffi] Fix incorrect kernel type for size in FFI transform.

Tested: Broke FFI implementation in dart2wasm
Change-Id: I45a7a194ea66c960a0fad00d7d5826b614159f7c
Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-linux-debug-x64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237685
Reviewed-by: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Commit-Queue: Aske Simon Christensen <589eb3fd9a84f06d0ac46d3aa71dddcfb0814ccd@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