A few of the dominant champions in top lane right now are Gragas, Lulu, Nidalee, and Maokai (not sure about solo Q, but he has been seeing LCS play). All of these champions deal a lot of magic damage (even AD Nidalee, since all her spells deal magic damage) If you're playing an melee champion top, unless you have some way of harassing at range and trading, you're going to get poked down. Null magic mantle is the only MR item you can afford lvl 1, and you're probably not going to build it into anything early (if at all), also buying a negatron cloak early really slows down your build (unless you're Mundo or something, in which case you can probably just go for Spirit Visage).
Really the only reason I bring it up is because Doran's shield isn't a universal tank item, it's only efficient against champions that auto attack a lot and where you don't need the extra potions. Against AD champions you do have the option of going cloth 5, but vs AP champions, you can go null magic 2, but you're going to take more harass from these ranged AP champions and you only have two potions to deal with it. The only other options are going for like, Dorans blade 1 pot and hoping you can chunk them early so they're scared to go in on you, or go for something super defensive like flask 3 pots, and just hope you don't take a lot of harass and can freeze lane in front of your turret, probably have to rely on jungle help too.