code: function setup (index) {
    function get () {
      return this[ArrayIndex.get](index);
    }
    function set (v) {
      return this[ArrayIndex.set](index, v);
    }
    return {
      enumerable: true,
      configurable: true,
      get: get,
      set: set
    };
  }
docs: Returns a property descriptor for the given "index", with "get" and "set"
  functions created within the closure.
  
  @api private