Dart's been very stable for a while now, and has a really good library ecosystem already. Personally, I wouldn't worry too much about JavaScript frameworks evo

Dart's been very stable for a while now, and has a really good library ecosystem already. Personally, I wouldn't worry too much about JavaScript frameworks evolving rapidly, just go with anything the supports Web Components. In JavaScript that would be Polymer, X-Tags or Bosonic. In Dart that's Polymer. Or, in either you can write directly to the platform APIs without a library. In my mind there is BWC and AWC: Before Web Components and After Web Components. The world or web apps is undergoing a paradigm shift on par with AJAX right now, and I wouldn't use any framework that doesn't support Web Components.