I have experimented a lot with Dart, both client and server side. I nice language and ecosystem. Smart move concentrating on compilation to JavaScript.

Clojurescript has been my favorite client side language for a while, but I am considering jumping to Typescript - a really well designed and implemented language.

It really doesn't seem to matter which language one writes in that can be transpiled to JavaScript. Pick one you like or just use plain JavaScript. This seems to be one of those cases where you can just pick what you like - a different situation than choosing between Java or C# or Haskell, etc.