fs: make fs.promises non-enumerable

This prevents the experimental feature warning from being emitted
in cases where fs.promises is not actually used.

PR-URL: #20632
Fixes: #20504
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

lib/fs.js

@@ -79,7 +79,7 @@ let warn = true;

Object.defineProperty(fs, 'promises', {
  configurable: true,
  enumerable: true,
  enumerable: false,
  get() {
    if (warn) {
      warn = false;

test/parallel/test-fs-promises.js

@@ -5,7 +5,8 @@ const assert = require('assert');
const tmpdir = require('../common/tmpdir');
const fixtures = require('../common/fixtures');
const path = require('path');
const fsPromises = require('fs').promises;
const fs = require('fs');
const fsPromises = fs.promises;
const {
  access,
  chmod,
@@ -38,6 +39,10 @@ const tmpDir = tmpdir.path;

common.crashOnUnhandledRejection();

// fs.promises should not be enumerable as long as it causes a warning to be
// emitted.
assert.strictEqual(Object.keys(fs).includes('promises'), false);

{
  access(__filename, 'r')
    .then(common.mustCall())