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 don't think that we should rush an impl unless some code depends on it - IMO it is preferable not to put add code to the writer and have a proper code transformer, then destructuring can be handled by adding an other transformer.

Could the `Map` case be solved with `for([k, v] of MapWrapper.iterator(map))` ?