[streams] Warning misbehaving underlying sources (#298)

(Branched from https://github.com/whatwg/streams/pull/296#discussion_r26482675)

Should we throw if an underlying source looks misbehaving?

Example: Underlying source calling close function provided by the stream twice or more

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/298