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 
Sure, I agree it would be possible to implement and I think it's a valuable use case to support.  In order to properly support it though I think we'd have to provide some level of a custom runtime like Traceur provides for ES5.  For instance, I think we would need a custom Map implementation that provides iteration of key and value like the ES6 Map.  I think it would also require ES6 generator support to make it really useful since that's a common example provided.

I wasn't really sure what the immediate vs long term goals were so I was just implementing the basic support to allow the simple case of for..of.