fs: lazy load the promises impl

PR-URL: #20766
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/fs.js

@@ -60,7 +60,6 @@ const { Readable, Writable } = require('stream');
 const EventEmitter = require('events');
 const { FSReqWrap, statValues, kFsStatsFieldsLength } = binding;
 const { FSEvent } = process.binding('fs_event_wrap');
-const promises = require('internal/fs/promises');
 const internalFS = require('internal/fs/utils');
 const { getPathFromURL } = require('internal/url');
 const internalUtil = require('internal/util');
@@ -89,14 +88,18 @@ const {
   CHAR_BACKWARD_SLASH,
 } = require('internal/constants');
 
-let warn = true;
+// Lazy loaded
+let promises;
+
+let promisesWarn = true;
 
 Object.defineProperty(fs, 'promises', {
   configurable: true,
   enumerable: false,
   get() {
-    if (warn) {
-      warn = false;
+    if (promisesWarn) {
+      promises = require('internal/fs/promises');
+      promisesWarn = false;
       process.emitWarning('The fs.promises API is experimental',
                           'ExperimentalWarning');
     }