Emit native ES6 classes with --target ES6

[danquirk]

class A { foo: string; }
class B extends A { bar: string; }

compiling with --target ES6 currently still uses our own __extends and not ES6 classes.

[paleo]

If a class is located in an internal module, is it possible to generate an ES6 class?

[DanielRosenwasser]

@Tarh classes are just expressions in ES6, so there shouldn't be a problem as far as I can tell.

[yuit]

The feature is implemented and pull into master. PR #2333