Backport of the fix to #17276 for Go 1.7.

Change-Id: Ifc1a8e2a81d4e543dbef04566985618884a8c0e0
Reviewed-on: https://go-review.googlesource.com/35635
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>