Error: /jonas/WebSockets package - UnusedBytecodeStripper2

It seems that Unity 5.0 final has a bug where il2cpp fails compiling the websocket-sharp.dll included in the project to make it work in the editor. I believe that the il2cpp team has already fixed this bug, to be shipped in a 5.0 patch release some time soon.

But you don't have to wait for that. The websocket-sharp.dll is only needed to make web sockets work in the editor and other platforms, and not actually needed on WebGL. So, simply select Plugins/websocket-sharp.dll in the editor, and, in the inspector, uncheck "Any Platform", and instead, check every platform *other then WebGL*. This will exclude it from the WebGL build (where it is not needed), and fix this error.