Off the top of my head, I imagine you'd create a Slider object with a min/max value of 0/1 and bind its value to the opacity of the layer, something like...
Slider{
id: slider
minimumValue: 0
maximumValue: 1
}
ArcgisTiledMapServiceLayer{
opacity: slider.value
}
In theory, as you alter the value of the slider, it should adjust the opacity of the layer.