R=go1.12

Update #26469

Change-Id: Iad75edfc194f8391a8ead09bfa68d446155e84ac
Reviewed-on: https://go-review.googlesource.com/127055
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>