we check length of the map, and a non empty map forces source mode.
Change-Id: Id314ad87b96d9154198122586fd8672a70c5ca95
Reviewed-on: https://go-review.googlesource.com/c/153198
Run-TryBot: Ian Cottrell <iancottrell@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>