Gotta ask. Why do you have a 12K line JS file?
The file in question is for this class http://jgraph.github.io/mxgraph/docs/js-api/files/view/mxGra....
1) It's the main public API to the the library, there's no logical way to split it.
2) We'd have a couple of thousand, very-annoyed-cos-they-paid-quite-a-lot-for-it customers, frankly, go apeshit if we split the file and broke their app on the next update because we wanted to switch IDE...
(I'm not trying to be snarky and I apologize if the above comes off that way.)
I for one think that developing a project in multiple files that can all be understood on their own is the only sane way to develop at all, regardless of editor choice.