Found by running the go vet pass 'testinggoroutine' that
I started in CL 212920.

Change-Id: Ic9462fac85dbafc437fe4a323b886755a67a1efa
Reviewed-on: https://go-review.googlesource.com/c/go/+/213097
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>