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>

Author: BridgeAR

lib/net.js

@@ -75,13 +75,12 @@ const {
 ERR_SOCKET_BAD_PORT,
 ERR_SOCKET_CLOSED
 }=errors.codes;
-constdns=require('dns');
 
 constkLastWriteQueueSize=Symbol('lastWriteQueueSize');
 
-// `cluster` is only used by `listenInCluster` so for startup performance
-// reasons it's lazy loaded.
-varcluster=null;
+// Lazy loaded to improve startup performance.
+letcluster;
+letdns;
 
 consterrnoException=errors.errnoException;
 constexceptionWithHostPort=errors.exceptionWithHostPort;
@@ -1034,6 +1033,8 @@ function lookupAndConnect(self, options) {
 thrownewERR_INVALID_ARG_TYPE('options.lookup',
 'Function',options.lookup);
 
+
+if(dns===undefined)dns=require('dns');
 vardnsopts={
 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) {
 };
 
 functionlookupAndListen(self,port,address,backlog,exclusive){
+if(dns===undefined)dns=require('dns');
 dns.lookup(address,functiondoListen(err,ip,addressType){
 if(err){
 self.emit('error',err);