Bottom line, Its very safe to say that dart is a much better language than JS.
The language of choice has never ever, really mattered in software history.
And don't forget that the big G is behind this thing, which is well equipped to push it forward without the need of it being as popular as js.
Right now Flutter/Dart looks like an ongoing political war between ChromeOS, Android and Fuchsia.
By the way, Fuchsia is getting Android suport and a new language agnostic UI composition engine, Scenic, so lets see for how long Flutter stays as the Fuchsia official UI toolkit.
But isn't the point of a truly cross platform framework just that? So what if fuschia has a different native toolkit. Maybe they are trying to launch a platform specific api that's more bare metal.
I mean Isn't the whole point of flutter is that you take the same app and run it anywhere?