[streams] Define the streams interface with only publicly visible behavior (#312)

We've been sporadically saying that we need a definition of the interfaces not by JS code but by describing publicly visible behavior of each property.

I'm trying to build it at:
https://github.com/whatwg/streams/blob/asyncbytestream/BinaryExtension.md

IIRC, @domenic said that the current spec also includes some texts doing this. But I couldn't find it.

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/312