Is that helper in the stream module pre-v6? v4 tests are failing.

Fixed, but the 6.8.1 failure seems fishy given nodejs/node#9088. In any case, that failure would be present regardless of this change since the instanceof check is exactly the same in isstream.

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Thank you for your understanding. If you have any questions, please check out #2556 and feel free to comment / ask anything there.