code: function(rec, prevChild) {
            var i;
            if( prevChild === null ) {
              i = 0;
            }
            else {
              i = this.$indexFor(prevChild)+1;
              if( i === 0 ) { i = this.$list.length; }
            }
            this.$list.splice(i, 0, rec);
            this._indexCache[this.$$getKey(rec)] = i;
            return i;
          }
docs: Used to insert a new record into the array at a specific position. If prevChild is
  null, is inserted first, if prevChild is not found, it is inserted last, otherwise,
  it goes immediately after prevChild.
  
  @param {object} rec
  @param {string|null} prevChild
  @private