Add missing require('fs')

lib/net.js

@@ -1094,7 +1094,7 @@ Server.prototype.listen = function () {
         if (!r.isSocket()) {
           throw new Error("Non-socket exists at  " + path);
         } else {
-          fs.unlink(path, function (err) {
+          require('fs').unlink(path, function (err) {
             if (err) throw err;
             self._doListen(path);
           });