A write completion callback that a JavaScript module returned reaches
the stream lowering as an untyped value. Since the error-argument form
of write callbacks, the lowering refused it with SC2020, so
tests/corpus/8532-stream-saved-callback-receiver failed to compile.
Such a callback is now converted to a function of the error argument.
The stream calls it unbound with null after a successful write or with
the write's error, and ignores what it returns, as Node does.
