[whatwg/fetch] ReadableStream and fetch/event-loop integration question (#976)

I have question regarding the use of streams in fetch, in the specific case of [`transmit the body`](https://fetch.spec.whatwg.org/#concept-request-transmit-body) of a request. 

So I think in all cases, when "transmit the body" is called, we are on "in-parallel" steps of the spec, hence I think we shouldn't be directly interacting with javascript objects, as described in https://html.spec.whatwg.org/multipage/#event-loop-for-spec-authors:in-parallel

So I'm wondering how, when transmitting the body of a request from parallel steps, the spec then says to:

1. [get a reader for the stream](https://fetch.spec.whatwg.org/#concept-get-reader)
2. Start [reading chunks](https://fetch.spec.whatwg.org/#concept-read-chunk-from-readablestream) from it until it is done or errored.

Wouldn't this require interacting with the corresponding javascript stream, which could consists of an arbitrary number of other streams piped/transformed into it?

For example, let's say the JS wants to upload some data to a server, which would be the end-result of a pipeline of streams. When "transmitting the body" of the request, we would effectively have to pull chunks from that pipeline and therefore run the corresponding javascript on a event-loop, correct? 

Would it perhaps be more correct to queue a task on the event-loop which would start reading chunks from the stream, and for each chunk queue "in-parallel" steps back on fetch that would transmit those chunks over the network?

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