spec, runtime, tests: send on closed channel panics

Close of closed channel panics.
Receive from closed channel never panics,
even if done repeatedly.

Fixes #1349.
Fixes #1419.

R=gri, iant, ken2, r, gri1, r2, iant2, rog, albert.strasheim, niemeyer, ejsherry
CC=golang-dev
https://golang.org/cl/3989042
4 files changed