Allow transferring FileSystemWritableFileStream?

[saschanaz]

I found that FileSystemWritableFileStream is not transferable, can we make it so? I see no blocker as FileSystemWritableFileStream doesn't seem to hold any additional field, so it should be as straightforward as just using the same transfer steps of WritableStream. Or is there some blocker against that?

Edit: maybe yes, because the implementation detail of how the file stream shuts down (on worker termination etc.) will then be visible. The spec should describe what should happen on termination.