What would happen if I deleted it?
I just tried building both beta 6 and beta 7 examples for WebGL in Unity 5.0.1 and they built fine. Are you building just the standard Fungus & FungusExample files or is this your game project. One possibility might be if you're using the Reorderable List control in a class that should only be built for the Editor but is actually getting build for the Editor and runtimes. You can make a class only build for the Editor by putting it inside an Editor folder in your project. That's only a guess though! If you're still stuck send me on the project and I'll try to build it here.
Exact same project, shared in Dropbox builds to WebGL fine on a Mac. I've made a new empty project and I added Fungus first and it built again. There are a few other external packages to add as well, then I'm adding my own project back piece by piece. Working so far