Once we've evicted all the blocked I/O, the ref count
should go to zero quickly, so it should be safe to
postpone the close(2) until then.
Fixes#1898.
Fixes#2116.
Fixes#2122.
R=golang-dev, mikioh.mikioh, bradfitz, fullung, iant
CC=golang-dev
https://golang.org/cl/5649076
Filed issue 3016 to fix this, but I really want
to see a "ok" in the Windows column so we
know what is and is not working.
R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5658050