[whatwg/streams] Simplify [@@asyncIterator].next() (#995)

See [this comment](https://github.com/whatwg/streams/pull/994#discussion_r258917942) on #994.

- Assert that getting `value` and `done` from `result` cannot throw.
- Rearrange the steps slightly to reduce redundant work.


You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/995

-- Commit Summary --

  * Simplify [@@asyncIterator].next()

-- File Changes --

    M index.bs (4)
    M reference-implementation/lib/readable-stream.js (2)

-- Patch Links --

https://github.com/whatwg/streams/pull/995.patch
https://github.com/whatwg/streams/pull/995.diff

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