I wish we could retrieve the solution we used for past levels. :(

Okay, I think I've replicated it. I just overrode onCollision with empty braces, so the final definition looked like this:

    map.defineObject('attackDrone', {
        'type': 'dynamic',
        'symbol': 'd',
        'color': 'red',
        'onCollision': function (player) {
            player.killedBy('an attack drone');
        },
        'behavior': function (me) {
        },
        'onCollision': function (player) {
        }
    });

Note that you're closing the game's braces, inserting another definition, then re-opening braces to match the game's closing ones.