function FocusEvent(ev) {
    // [Constructor(DOMString typeArg, optional FocusEventInit focusEventInitDict)]
    // interface FocusEvent : UIEvent {
    //     readonly    attribute EventTarget? relatedTarget;
    // };

    UIEvent.call(this, ev);
}
See [UI Events (formerly DOM Level 3 Events)](http://www.w3.org/TR/2015/WD-uievents-20150428/#events-focusevent).

@class FocusEvent
@augments UIEvent

@param {Event} ev The native DOM event.