I hear you, but what do you expect Dart's contribution as a language/technology to be in this space, effectively? While syntactically comparable, TypeScript is a better typed JavaScript than Dart will probably ever be (in terms of leveraging a type system to enable more complex/maintainable apps and libraries). And since every programming language compiles to JavaScript these days, you've got some compelling options if you really want to depart from JS altogether (with http://www.scala-js.org/ an amazing one, for which I find the mix of Functional + Object Orientation to lend itself very well to the web).
In all, I really fail to see a good reason for Dart to exist nowadays (and surviving as "Flutter's best language buddy" isn't a great one).