Trying to understand the linux-386-387 failures:
http://build.golang.org/log/78a91da173c11e986b4e623527c2d0b746f4e814

Also modernize the closeOnce code with a method value, while I
was looking.

LGTM=adg
R=golang-codereviews, adg
CC=golang-codereviews, iant
https://golang.org/cl/87950044