After the mission 'Politics', where Johnny is recruited to assassinate someone for Congressman Stubbs, the news reports that the target was shot 'Shortly after his plane landed.' However, the target arrived by helicopter, and can be killed by rocket launcher as he lands.
In the opening cutscene Niko Bellic can be seen walking down a street as Johnny Klebitz and The Lost ride through Alderney City. In Grand Theft Auto IV (2008), Alderney would have been blocked off at this point in the timeline, so there is no way Niko could have been there as he would have been in Broker at this time.