Discussion about the requirement can be read in the Trac ticket.
@fatherbushido suggested to follow this approach: "flaming projectile 'ignites' (or a poisonous one poisons) the target which suffers damage for a while regardless of running away."
Nice nomenclature defined on DOTA wiki which I'd follow in this patch.
This Diff is in progress. If you like the approach I can extend it with melee and ranged splash attacks. Sound or animation would be nice because right now it's not that obvious why the unit is being damaged over time.