[streams] Add IsReadableStreamDisturbed predicate (#385)

See the suggestion in the OP of #378.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/streams/pull/385

-- Commit Summary --

  * Add IsReadableStreamDisturbed predicate

-- File Changes --

    M index.bs (13)
    M reference-implementation/lib/readable-stream.js (12)
    M reference-implementation/run-tests.js (2)
    M reference-implementation/test/readable-stream.js (30)

-- Patch Links --

https://github.com/whatwg/streams/pull/385.patch
https://github.com/whatwg/streams/pull/385.diff

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https://github.com/whatwg/streams/pull/385