net: lazy load dns

PR-URL: #20567
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/net.js

@@ -75,13 +75,12 @@ const {
   ERR_SOCKET_BAD_PORT,
   ERR_SOCKET_CLOSED
 } = errors.codes;
-const dns = require('dns');
 
 const kLastWriteQueueSize = Symbol('lastWriteQueueSize');
 
-// `cluster` is only used by `listenInCluster` so for startup performance
-// reasons it's lazy loaded.
-var cluster = null;
+// Lazy loaded to improve startup performance.
+let cluster;
+let dns;
 
 const errnoException = errors.errnoException;
 const exceptionWithHostPort = errors.exceptionWithHostPort;
@@ -1034,6 +1033,8 @@ function lookupAndConnect(self, options) {
     throw new ERR_INVALID_ARG_TYPE('options.lookup',
                                    'Function', options.lookup);
 
+
+  if (dns === undefined) dns = require('dns');
   var dnsopts = {
     family: options.family,
     hints: options.hints || 0
@@ -1368,7 +1369,7 @@ function listenInCluster(server, address, port, addressType,
                          backlog, fd, exclusive) {
   exclusive = !!exclusive;
 
-  if (cluster === null) cluster = require('cluster');
+  if (cluster === undefined) cluster = require('cluster');
 
   if (cluster.isMaster || exclusive) {
     // Will create a new handle
@@ -1482,6 +1483,7 @@ Server.prototype.listen = function(...args) {
 };
 
 function lookupAndListen(self, port, address, backlog, exclusive) {
+  if (dns === undefined) dns = require('dns');
   dns.lookup(address, function doListen(err, ip, addressType) {
     if (err) {
       self.emit('error', err);