My kass build before DFG nerf was something like this
Bootspots > DFG > RoA > Sorc boots > Deathcap > Zhonyas/GA > Situational (Usually Lichbane).
I never really liked Tear on Kassadin because I feel he's such an offensive champion, if he doesn't do enough damage to push his enemy out of lane, then he isn't really doing his job correctly. Why not just zone out the enemy mid and use your Ws passive to regen mana? Also, your jungler should really be giving you blue :P
Late game his job is essentially to blow something quickly, so I don't know why you would need a high mana pool to do that. One combo of DFG > R > Q > E > W and the enemy something should be either DEAD, or unavailable to continue in the fight.
Now these days, I dont feel like rushing DFG is as good as it was so I might build something along these lines:
Bootspots > Kage Pick + Sorcerer boots > RoA > Deathcap > DFG > Zhonyas/GA etc etc.
TDLR : Tear prevents early game nuking and zoning, lategame you don't need the mana.