This is a backport of https://go-review.googlesource.com/30171.

Updates golang/go#7402.

Change-Id: I4e79570964c248ec52acf14d0ed8306e52073a51
Reviewed-on: https://go-review.googlesource.com/43212
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>