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 
I haven't look at the details but I believe that what works for ES5 should be easily applicable to Dart ?
Any particular concerns you can think of ?