add in todo for a better t.isReferenced

lib/6to5/types/index.js

@@ -127,6 +127,7 @@ t.isDynamic = function (node) {
   }
 };
 
+// todo: https://github.com/eventualbuddha/ast-util/blob/9bf91c5ce8/lib/index.js#L454-L507
 t.isReferenced = function (node, parent) {
   // we're a property key and we aren't computed so we aren't referenced
   if (t.isProperty(parent) && parent.key === node && !parent.computed) return false;