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 
<code>x = $__0[0], y = $__0[1];</code> as far as I can tell is not valid Dart code.