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

> I don't quite follow the reasoning here. If we want to exert back-pressure we can change transform.writable.state. If we want to signal success/failure of the transform, we use the return value of transform.writable.write(). What am I missing? Code might help :)

Ah, I was talking a subtle thing about the interface between the sink and the WritableStream. `underlyingSink.write()` is invoked by `WritableStreamAdvanceQueue`. Once `underlyinSink.write()` fulfills, it continues to advance more (step "g"). So, `underlyingSink.write()` alone cannot exert back-pressure without delaying fulfillment of ws.write().

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