The way I see this going down is late TKO/KO or Mikey UD.
There's so many 'if's' with Broner, the only 'if' with Mikey is can he take Broners punch, IMO he'll be able to no problem. He's steadily in the gym, and will adapt to 140 (bringing up his power, having his chin tested) without an issue. Mikey's also a sharp puncher with heavy hands, he has sneaky power, and disguises his shots well. I think Broner will feel this a lot in left hooks, and counter check hooks the most. Likewise, I think Mikey will be able to outbox Broner, with his ++ jab pumping all night, and superior positioning keeping him one step ahead of Broner, while also negating the handspeed and size advantage.
Howeverrrrrrrrrrrrrrrrrrrrrrr, Adrien Broner is extremely talented, with a wide skill set. He has fast hands, fast feet, good offensive skills, and good defense, also his power is + at 140. If he gets down right, and is extremely focused, he can give Mikey a hell of a fight. Mikey, despite having great timing, doesn't have the fastest hands. If Broner is able to consistently get his jab off first, then start hooking off the jab, the fight could turn his way---although I doubt it.
There are many other variables, but this is a quick version of my thoughts.