http://golang.org/cl/50251 fixed a regression under TSAN.
This change adds a minimal reproducer for the observed symptom.

Change-Id: Ib9ad01b458b7fdec14d6c2fe3c243f9c64b3dcf2
Reviewed-on: https://go-review.googlesource.com/50371
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>