For a long time, the debugger connection has had a `_serverConnection` property that allowed you to grab the "other end" of the connection when e10s is off, which made it easier to write certain tests. After bug 1240907, we now use the message manager for all tabs in DevTools for both e10s off and on. This means `_serverConnection` really isn't useful anymore. We should restructure tests that use this and remove the property.