Support ES6

[serjek]

es5 compiles just fine, but -D js-es=6 causes following compile error:

/haxe_libraries/coconut.ui/0.11.2/github/a2987279f3b189e5a02d31a46837d765250c7ca5/src/coconut/ui/macros/ViewBuilder.hx:797: characters 20-77 : Must call `super()` constructor before accessing `this` in classes derived from an extern class with constructor

Having es6 support would be super-nice since most modern RN components have API and examples described in es6 already and it takes a while to debug them with current es5 output.

[back2dos]

@serjek @kevinresol I've made a relatively crude workaround that does seem to work with react-dom. Can you please confirm that this works with react-native? The haxe_libraries from coconut.ui should do the trick.