When they were traveling through the breach, there were various images flashing by.... one of those images was Supergirl in flight.
Although, I guess it doesn't need to be another Earth, because it looks like images from other times flashed as well.
The images that flashed by:
Supergirl
Green Arrow w/goatee, confirmed to be appearing in Legends of Tomorrow (from the future)
Jonah Hex, confirmed to be appearing in Old West episode of Legends of Tomorrow
Legion of Super-Heroes flight ring
The 1990 Flash