The source for the plugin is not currently available. Even if it was, the eclipse plugin is built on top of the 3rd-party DLTK plugin(s) and the eclipse plugin framework, which makes the vast majority of the current plugin code irrelevent in the intellij plugin framework - the only notable exceptions would be the core syntax portion of the parser (which is less than 1/2 of the overall parsing process) and the SSDP-based discovery code (which is just a convenience for the console and the deployment wizard).
I had been wondering about that same question just yesterday.
First I had quite a few questions about the plugin. From reading previous forum threads I got the impression that it can be difficult to get support for the eclipse plugin. Thus, I decided to have a look at the plugin internals myself. Not only did this answer those questions I had, I was even able to identify two bugs in the code within a very short time.
So, I'm asking just like the others: Couldn't you put the code on GitHub?
You got nothing to loose but a lot to win by doing so...