Found with GODEBUG=wbshadow=2 mode.
Eventually that will run automatically, but right now
it still detects other missing write barriers.

Change-Id: Iea83d693480c2f3008b4e80d55821acff65970a6
Reviewed-on: https://go-review.googlesource.com/2277Reviewed-by: Keith Randall <khr@golang.org>