This was supposed to be in CL 18205 but I submitted via the web
instead of from my computer, so it got lost.
May deflake some things.

Change-Id: I880fb74b5943b8a17f952a82639c60126701187a
Reviewed-on: https://go-review.googlesource.com/18259Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>