Javascript is very different to objectif-c or Swift, because of the event-driven nature. So a wrapper for Javascript in Objectif-C is not a real option.
If you want to port the library to IOS you simply need to open a HTTP Request and consume the Results. But keep in mind: you have to do the Asynchronus stuff by yourself.
The Eventsource (at a low level) is simply a pending HTTP Request, producing output if an event arrives.