syncmap: backport CL 42095

See https://go-review.googlesource.com/c/42095/.

Change-Id: I9f15a785bb4bd28ab02f17de65c63ad4b6e083c9
Reviewed-on: https://go-review.googlesource.com/42594
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
1 file changed