"Wait until any ongoing write finishes (i.e. the..."

[ricea]

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.

[domenic]

or the promises equivalent to those that would have been returned from writer.write() if we were using the public API.

This was the intent. In the reference implementation it was implemented using WritableStreamDefaultWriterWrite, but I guess we never called that out by name. Probably we should...