property altKey

This property returns a boolean which will be true if the Alt key is pressed when the event happens.

<box on:keyPress="onKeyPress();"/>

// KeyEvent is inherited from UIEvent
function onKeyPress()
{
    var eventObj = shell.currentEvent;
    if (eventObj)
    {
        if (eventObj.altKey)
        {
            shell.print("Alt key is down");
        }
        else
        {
            shell.print("Alt key is not down");
        }
    }
}