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 
My last comment was stating that the MultiVarTransformer wouldn't transform <code>x = $__0[0], y = $__0[1]</code> into <code>x = $__0[0]; y = $__0[1]</code> since it's not a variable declaration list.  I thought about writing a <code>CommaExpressionTransformer</code> but didn't think of overriding <code>transformList</code> to support returning an array from <code>transformCommaExpression</code>.  I could go that route instead to support the general comma expression transformation.  The tricky part though is knowing where the multiple statements would still be valid.  I figured in this particular scenario it was safe to re-write without impacting anything.