Starts the hide animation.
function _startHideAnimation(item, prevItem, size) {
        var spec = item.options.hide.animation ? item.options.hide.animation.call(undefined, false, size) : {};
        item.endSpec = spec;
        item.startSpec = {
            opacity: 1,
            transform: Transform.identity
        };
        if (!item.halted) {
            item.mod.halt();
            var callback = item.hideCallback;
            if (spec.transform) {
                item.mod.setTransform(spec.transform, item.options.hide.transition, callback);
                callback = undefined;
            }
            if (spec.opacity !== undefined) {
                item.mod.setOpacity(spec.opacity, item.options.hide.transition, callback);
                callback = undefined;
            }
            if (callback) {
                callback();
            }
        }
    }