* Pass along the original property names to
  binding so we don't have to disown the
  existing ones and add the new ones.
* Although it is a delicate matter:
  //perfectionkills.com/understanding-delete/
  avoiding delete where you can is better for
  JS optimizers IMO.
  (hint: it de-optimizes the whole object to
   change its internal "shape", which means
   delete single property from object literal
   comes with a perf. trade-off)

PR URL: #727.