net: require 'dns' only once

Avoid unnecessary calls to require('dns') by caching the result of the
first one.

PR-URL: #12342
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

lib/net.js

@@ -40,8 +40,9 @@ const PipeConnectWrap = process.binding('pipe_wrap').PipeConnectWrap;
 const ShutdownWrap = process.binding('stream_wrap').ShutdownWrap;
 const WriteWrap = process.binding('stream_wrap').WriteWrap;
 
-
 var cluster;
+var dns;
+
 const errnoException = util._errnoException;
 const exceptionWithHostPort = util._exceptionWithHostPort;
 const isLegalPort = internalNet.isLegalPort;
@@ -970,7 +971,7 @@ function realConnect(options, cb) {
 
 
 function lookupAndConnect(self, options) {
-  const dns = require('dns');
+  const dns = lazyDns();
   var host = options.host || 'localhost';
   var port = options.port;
   var localAddress = options.localAddress;
@@ -1309,6 +1310,13 @@ function emitListeningNT(self) {
 }
 
 
+function lazyDns() {
+  if (dns === undefined)
+    dns = require('dns');
+  return dns;
+}
+
+
 function listenInCluster(server, address, port, addressType,
                          backlog, fd, exclusive) {
   exclusive = !!exclusive;
@@ -1437,7 +1445,7 @@ Server.prototype.listen = function() {
 };
 
 function lookupAndListen(self, port, address, backlog, exclusive) {
-  const dns = require('dns');
+  const dns = lazyDns();
   dns.lookup(address, function doListen(err, ip, addressType) {
     if (err) {
       self.emit('error', err);