- This was not a premonition or a dream
- I believe that the "dream sequence" was actually memories from the Flash that came back in time.
- Flash was not referring to Superman as the "him" in this instance, possibly as he was in an alternate future.
So from the DC movies, although I am sure they have a general road map penciled down on a napkin somewhere, they are doing their damnedest to keep us all in suspense and not explaining very much.
Comparing that "dream sequence" to the events of Justice League:
The only similar aspect is the winged creatures themselves. Lois is still alive, Batman does not have an army, Superman is not evil, there are no giant Omega symbols on the ground, etc. The two events are not related.
More likely the "dream" was not that, but it was a memory or something transferred from the Flash onto Bruce somehow. Obviously Flash came back in time to warn Bruce that he was right about someone. Given the title of the movie, Batman v. Superman, the obvious connection is that Bruce is right about Superman and he is doing the right thing trying to kill him.
But we have no other context about the scenes other than the images we see. Who is to say that Flash is warning Bruce about Superman? Even Flash questions whether or not he arrived at too early a point. Bruce also has no clue who the Flash is at that point. Maybe 'him' is actually some other villain that we haven't even met yet.
The biggest easter egg we learn IMO from the "vision" is the omega symbol. The omega symbol is more than likely (if not obviously) a reference to Darkseid. My money is on the entire "vision" that Flash shares, just as he suspects, as having been WAY to early and more than likely it will not even relate until the next Justice League movie; if ever.
From what we have seen so far Clark goes nutso when someone threatens either his mom or his girlfriend. Flash telling him that Lois is the key is extremely important. Related to the Justice league movie:
Bruce even recognizes that Lois is "the big gun" to calm him down.
So Bruce now understands the importance both of the women play in Clark's life. In the vision even Superman essentially says that without "her" he is lost and that is why he has gathered his little army and gone on a rampage. Given this, it is possible that Flash was indicating that Bruce was right about Superman; that he is the one too be worried about.
But, now that Bruce knows how twitchy Clark gets about the women in his life, he may be able to protect them and keep those events he 'foresaw' from ever happening.
Note: I am the furthest from knowing everything about the DC comic book universe, but I did a bunch of research about this after I first saw BvS because the scene had so much packed into it and I was so confused.