[streams] TransformStream: transform() is never invoked via pull() (#330)

When `write()` of the underlying sink of `writable` is called, `maybeDoTransform()` always proceeds to run `transform`. It never stop with `writeChunk` pending but `transform` not started. Maybe what we want to do is not to start `transform` while there's back-pressure on `readable`.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/330