net: refactor onSlaveClose in Server.close

Refactors onSlaveClose in Server.close to be an arrow function,
removes need for `self = this` and moves it down to make code
more readable.

PR-URL: nodejs/node#12334
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>

lib/net.js

@@ -1495,13 +1495,6 @@ Server.prototype.getConnections = function(cb) {
 
 
 Server.prototype.close = function(cb) {
-  function onSlaveClose() {
-    if (--left !== 0) return;
-
-    self._connections = 0;
-    self._emitCloseIfDrained();
-  }
-
   if (typeof cb === 'function') {
     if (!this._handle) {
       this.once('close', function() {
@@ -1518,8 +1511,13 @@ Server.prototype.close = function(cb) {
   }
 
   if (this._usingSlaves) {
-    var self = this;
     var left = this._slaves.length;
+    const onSlaveClose = () => {
+      if (--left !== 0) return;
+
+      this._connections = 0;
+      this._emitCloseIfDrained();
+    };
 
     // Increment connections to be sure that, even if all sockets will be closed
     // during polling of slaves, `close` event will be emitted only once.