I love Flutter, but I'm still slightly reluctant to go for full adoption until I see a firm commitment to Dart. If Google makes Dart the main SDK language for Android (like Swift on iOS), that'll do it!
The reply from a Google employee when asked this question was to look at the Google AdWords team - they have invested heavily in Flutter for their Google Ads app on iOS and Android. He basically said Flutter will be supported for 10+ years as a minimum.
The AdWords team is the one that rescued Dart 1.0 project from being completely canceled, after the Chrome and Angular teams no longer cared.
I've worked quite a bit with Dart on my time the past year and it was great. Being a JS/TS developper I found my mark pretty quickly. However as it's not _that_ popular and the online resources (be it SO questions or articles) are a bit scarce and the most recent one are usually centered around Flutter and not just Dart.
I'll keep playing with it and if the possibility would occur I'd happily work full time with Dart.
Google is moving away from Android, the replacement has Dart built-in.
Google is porting Android to run on Fuchsia as well, similar to ChromeOS.
Fuchsia tries to be language agnostic thanks to FIDL. Dart will be one of many options.
What is the "replacement"?
Not going to happen, considering Google's commitment to supporting Kotlin for Android development over the past couple of years.