feat(transpiler): Transform for..of to Dart as for..in

Added transformation of for..of into Dart for..in.  
Added error for JS for..in since the semantics don't match.  
Added error for for..of utilizing destructuring declarations.

Closes #53 
@tpodom do you have time to work on a more generic solution as described in my last comment ?
- transpile for..of to for..in in Dart,
- add a destructuring transformer (based on what exists for es6 -> es5)