When you say the "dev compiler" are you referring to the DartVM? While (embedded in Dartium browser) that's used for dev convenience for front-end, it's also (without the browser, obviously) the main target for back end Dart.
The dev compiler translates Dart code into human readable JS code. So you can write a library in Dart and still provide JS code someone could realistically adapt to their environment or if Dart were to go away it would allow you an easy path of escape.