dns: lazy loaded

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/dgram.js

@@ -35,7 +35,6 @@ const {
   ERR_SOCKET_DGRAM_NOT_RUNNING
 } = errors.codes;
 const { Buffer } = require('buffer');
-const dns = require('dns');
 const util = require('util');
 const { isUint8Array } = require('internal/util/types');
 const EventEmitter = require('events');
@@ -47,6 +46,9 @@ const { UV_UDP_REUSEADDR } = process.binding('constants').os;
 
 const { UDP, SendWrap } = process.binding('udp_wrap');
 
+// Lazy load for startup performance.
+let dns;
+
 const BIND_STATE_UNBOUND = 0;
 const BIND_STATE_BINDING = 1;
 const BIND_STATE_BOUND = 2;
@@ -72,9 +74,10 @@ function lookup6(lookup, address, callback) {
 
 
 function newHandle(type, lookup) {
-  if (lookup === undefined)
+  if (lookup === undefined) {
+    if (dns === undefined) dns = require('dns');
     lookup = dns.lookup;
-  else if (typeof lookup !== 'function')
+  } else if (typeof lookup !== 'function')
     throw new ERR_INVALID_ARG_TYPE('lookup', 'Function', lookup);
 
   if (type === 'udp4') {