core-list start item at specyfic index

[vito...@gmail.com]

When script load, how to set start item on index (ex. 20)

core-list.data = has 100 items in array
start at index 20

I try

ready: function() {
    this.$.list.scrollToItem(20);
}

whit error result: this.setScrollTop is not a function

on domReady looks like some issue when scrolling up

onClick work perfect but I need set the index automatically when page load.