Context: I'm using Dart for app development for two years now.

> Not all language features can be represented in JavaScript.

I could not name one (except the dart:io stuff, which is not supported in the JS world). Could you please elaborate?

> Besides, all the languages that compile to JavaScript just add complexity layers to the already complex model of HTML/CSS/JavaScript.

I've found the same, but Dart is a surprising exception. If you are not using any fancy framework (e.g. Angular.Dart), you will be just fine with the close-to-the-browser approach of the language and APIs.