Like FmtFlag constant in fmt.go

Change-Id: I351bcb27095549cf19db531f532ea72d5c682610
Reviewed-on: https://go-review.googlesource.com/c/go/+/209497
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>