I have several humongous projects in Dart and have been using it for around 5 years now. I would still choose it over a regular NPM setup, although that recently changed with KotlinJS maturing a bit more.
Dart2 which finally got rid of the wishy-washy optional types and went to inferred types really makes it a great language. It's still missing null-safety though, but otherwise it's like a very modern version of Java minus most of the horrible stuff in Java.
I've also not jumped on the Flutter bandwagon, I'm confident that just like with with Phonegap, Cordova, Ionic, ReactNative, Xamerin and many others, you'll hit some of the same problems with Flutter eventually when building enormous projects, maybe less of them, but running anything inside a container will always be less performant with some features unavailable due to the abstractions.
Both Dart and Flutter are used internally at Google to build AdWords and the AdWords mobile app, so it's not going to go away anytime soon. The risk is obviously there if Google decides to no longer use Dart internally, then it'll become abandonware, but that same risk is there with Oracle ditching Java, Microsoft ditching TypeScript, etc