- Scala / Scala.js (http://www.scala-js.org/) and I'm actually using this one for a non-trivial app
- Clojure / ClojureScript (https://github.com/clojure/clojurescript) with some companies using it in production
Both are using Google's Closure compiler to do tree shaking minification and are emitting source maps for the compiled code, so the size of the compiled artifacts is manageable in spite of the huge standard libraries they come with and debugging works well. Normally if you want tree shaking with Google Closure, you have to write code specific for Google Closure, however these compilers are doing that for you.
I'm happy that compilers for C# are happening as well. My only gripe is that in true .NET fashion I'm not seeing a link to the source, which makes me think that this won't be open-source. Which is a pity.
With es6 around the corner and the possibilities those features will provide, i would still bet on pure js. Those precompilers will benefit a great deal too. Symbols and Proxies alone will remove a lot of the hoops they have to go through now.
I don't get this argument on the size of a hello world, given that JQuery 1.11 is over 270 KB and most people are just doing simple DOM selection with it, yet they don't complain their hello world comes with 270 KB of stuff they'll never use. Factor in React, Bootstrap, Angular.js or what have you and you can easily reach 1 MB of stuff that's never used for a couple of lines of code.
My current app is currently clocking at 300 KB btw, but I am using the collections from the standard library, I am using a library for reactive streams built by myself, I am using React and I do have some code in it. And personally I find it OK.