[js][es6] make this-before-super filter smarter

my own thoughts from slack:

> hmm, I wonder if I should do interprocedural check for this-before-super, sounds easy enough and if nothing accesses this, it should be safe to move field inits after the super call and not generate the hx_init magic, right?

> could even be smarter and check particular field access, not any 'this' access, but that sounds more complicated