https://github.com/WeTheInternet/xapi/tree/master/gwt/gwt-re...
If you want to support complete reflection across the board it greatly swells compile size, but I have made it so you can be selective about what supports reflection, in order to avoid code bloat.
How much do the threads actually work yet? How slow are they?
I'm not sure how the other transpilers expose native javascript, but in the future of Gwt, interacting with plain javascript is dead simple; you get the best of both worlds; typed java code, and low-level js performance. The JsInterop stuff makes wrapping native js super easy, just define an interface that matches the JS, and it "just works" (tm). Even web components, just create an annotated interface, define default methods, and the compiler attaches them to custom element definitions. I could go on and on, but I'll save it for the correct forum (i.e., the gwt.create conference). :D
Real threading would have to use ES6 yield/generators to translate an entire thread into a spawn loop so that async/await blocks can be emulated.