Open
Description
https://streams.spec.whatwg.org/#rs-pipeTo-shutdown-with-action
Wait until any ongoing write finishes (i.e. the corresponding promises settle).
It's not clear whether this refers to the promises returned by the underlying sink's write() method, or the promises equivalent to those that would have been returned from writer.write() if we were using the public API. This may make a difference depending on the resolution of #611.