Right now, the Mets do not have the necessary, big-time pitcher needed to get Dodgers OF Matt Kemp. The Dodgers want to win now. They've invested A LOT of money in this run, so they want a tested pitcher back (like Jon Lester) to help make it happen.
The buzz in LA is they'll deal Kemp because of his contract, history, he wants to play center field and that's not going to happen with the Dodgers. So, they'll pay a good portion of his remaining $100 million contract and give up a top prospect with him to get the pitcher they need to be put them over the top this October (Nightengale, July 29). And, the Mets just don't have that player to give right now... and, if they did, they'd rather keep him.
It's not Bartolo Colon, at least not in a deal for Kemp. I know the Dodgers like Jon Niese, but he has no playoff experience and this current shoulder situation is a bit of mystery. So I don't see a match.
Noah Syndergaard, Rafeal Montero, Jacob deGrom and Travis d'Arnaud are all really nice players, they're terrific prospects, but they're not going to help the Dodgers win a championship this year, which is their goal.
That said, if Kemp doesn't get moved this season (or through waivers in August), I believe the Dodgers will consider a package of future prospects for Kemp this winter, such as the Mets pitchers listed above. But, right now, they want a guy like Lester, like David Price, or both, which is why we keep seeing that rumor (and nothing with the Mets) over and over again...