dns: simplify dns.promises warning logic

dns.promises is lazy loaded. Instead of using a seaparate
Boolean flag to track whether or not it has been loaded, just
inspect the state of the lazy loaded module itself.

PR-URL: #24788
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

lib/dns.js

@@ -50,8 +50,7 @@ const {
 
 const dnsException = errors.dnsException;
 
-let promisesWarn = true;
-let promises; // Lazy loaded
+let promises = null; // Lazy loaded
 
 function onlookup(err, addresses) {
   if (err) {
@@ -269,7 +268,7 @@ function defaultResolverSetServers(servers) {
   setDefaultResolver(resolver);
   bindDefaultResolver(module.exports, Resolver.prototype);
 
-  if (promises !== undefined)
+  if (promises !== null)
     bindDefaultResolver(promises, promises.Resolver.prototype);
 }
 
@@ -318,10 +317,9 @@ Object.defineProperties(module.exports, {
     configurable: true,
     enumerable: false,
     get() {
-      if (promisesWarn) {
+      if (promises === null) {
         promises = require('internal/dns/promises');
         promises.setServers = defaultResolverSetServers;
-        promisesWarn = false;
         process.emitWarning('The dns.promises API is experimental',
                             'ExperimentalWarning');
       }