[typo] close the node-core WebSocket server

A little typo has been preventing the node-core WebSocket server from shutting down properly. (For the origin of the mis-typed property, see https://github.com/adobe/brackets-shell/blob/master/appshell/node-core/Server.js#L218) I'm not totally sure, but this may have been preventing the NodeConnection auto-reconnect stuff from working right because the WS server wasn't ever explicitly closing the sockets before shutting down or restarting.

TODO: add static types to JavaScript.