function traverseEnterLeave(from, to, fn, argFrom, argTo) {
	  var common = from && to ? getLowestCommonAncestor(from, to) : null;
	  var pathFrom = [];
	  while (from && from !== common) {
	    pathFrom.push(from);
	    from = from._hostParent;
	  }
	  var pathTo = [];
	  while (to && to !== common) {
	    pathTo.push(to);
	    to = to._hostParent;
	  }
	  var i;
	  for (i = 0; i < pathFrom.length; i++) {
	    fn(pathFrom[i], 'bubbled', argFrom);
	  }
	  for (i = pathTo.length; i-- > 0;) {
	    fn(pathTo[i], 'captured', argTo);
	  }
	}
Traverses the ID hierarchy and invokes the supplied `cb` on any IDs that
should would receive a `mouseEnter` or `mouseLeave` event.

Does not invoke the callback on the nearest common ancestor because nothing
"entered" or "left" that element.