Isn't Dart for browsers? And it looks like WebAssembly might overtake Dart there. Maybe that would work for Chrome Web Apps on Android, I haven't tried.
For native apps, I think it's still mainly Java, tho JetBrains is promoting Kotlin as an alternative.
The way I remember it dart was released to replace javascript as an actual viable sane language for apps, which means it could double as a longterm replacement for smartphone apps as well.
Mozilla resisted dart in the browser and focused heavily on improving javascript which left dart dead in the water.
For native apps, I think it's still mainly Java, tho JetBrains is promoting Kotlin as an alternative.