I'm no expert in the Android ecosystem, but I think it was primarily a good choice for giving devs features they wanted to have while the underlying runtime and standard library got stuck at Java 6. The reason why they are still on Java 6 might have to do with the law suit, though.
I guess given the recent court ruling we should see if Google sticks with Kotlin as a hedge, sticks with Kotlin because the community has embraced it, or pulls away from Kotlin as a no longer needed source of extra work.
It's Google. They'll probably come up with a completely new programming language which they'll force on the community in some ham fisted way, then suddenly drop it in a few years when they can't figure out a way to massively monetize it.
Dart always stuck me as a solution looking for a problem, particularly in lite of more recent progress in JS itself. Can I ask why you think its a genuinely great language worth looking at?
Not really. Dart is single-threaded which was a mistake Google made in trying to keep it JS-compatible. If Dart had been multi-threaded from the start it might have stood a chance.