A killer app is something like Rails. Rails made Ruby popular, because it was so good that people who otherwise wouldn't have been interested in Ruby were willing to use it for the benefits they got from Rails.
I suppose Flutter is Dart's killer app, since you're comparing frameworks and languages. Rails is to Ruby as Flutter is to Dart, as it were. I suggest you give it a try. Personally, this felt like the "killer" feature to me, fully advanced animations that I'm not sure are possible on native widget based frameworks: https://www.youtube.com/watch?v=FCyoHclCqc8
