This just overrides the (identical behaving) implementation from the base WritableStream class. Remove it from FileSystemWritableFileStream. WritableStream didn't use to have a close() method, as such FileSystemWritableFileStream implemented its own. Later WritableStream added one, but we forgot to remove the implementation of the override before shipping the API. This fixes that inconsistency between the spec and implementation.
Make the implementation match the spec. This change has no behavioral consequences.