function once(event) {
        return (listener, thisArgs = null, disposables) => {
            // we need this, in case the event fires during the listener call
            let didFire = false;
            let result = undefined;
            result = event(e => {
                if (didFire) {
                    return;
                }
                else if (result) {
                    result.dispose();
                }
                else {
                    didFire = true;
                }
                return listener.call(thisArgs, e);
            }, null, disposables);
            if (didFire) {
                result.dispose();
            }
            return result;
        };
    }
Given an event, returns another event which only fires once.

@param event The event source for the new event.