A modern programming language that makes developers happier.

kotlinlang

GWT is still being used by Inbox, Sheets, several other products at Google and AWS is also using it (they have several hour compile times at AWS due to the multiple languages being included) Super Dev Compiler makes things a lot faster, but you still have to write bulky Java and lots of boilerplate.
So far Dart has been the best experience, it's very close to JS (make change and refresh immediately), very fast, outputs well optimised code and it has an optional type system, so you can add types to the critical parts of the system and where it's not strictly needed, just use var.
Further, it has dead code removal, polyfills which make sure that if you use a Dart supported feature, it works in all browsers.
Packaging system which feels like NPM combined with Gradle, but only less clunky.
If a transpiler is used, one expects either a single JS file per browser (like with GWT) or polyfilled JS file (like with Dart) - if the output just maps straight to JS without making it cross browser compatible, then you might as well write pure JS.