This is just torazaburo's answer, updated to use a MouseEvent object.
function click(x, y)
{
var ev = new MouseEvent('click', {
'view': window,
'bubbles': true,
'cancelable': true,
'screenX': x,
'screenY': y
});
var el = document.elementFromPoint(x, y);
el.dispatchEvent(ev);
}