readline: 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/tty.js

@@ -26,9 +26,11 @@ const net = require('net');
 const { TTY, isTTY } = process.binding('tty_wrap');
 const errors = require('internal/errors');
 const { ERR_INVALID_FD, ERR_TTY_INIT_FAILED } = errors.codes;
-const readline = require('readline');
 const { getColorDepth } = require('internal/tty');
 
+// Lazy loaded for startup performance.
+let readline;
+
 function isatty(fd) {
   return Number.isInteger(fd) && fd >= 0 && isTTY(fd);
 }
@@ -122,15 +124,19 @@ WriteStream.prototype._refreshSize = function() {
 
 // Backwards-compat
 WriteStream.prototype.cursorTo = function(x, y) {
+  if (readline === undefined) readline = require('readline');
   readline.cursorTo(this, x, y);
 };
 WriteStream.prototype.moveCursor = function(dx, dy) {
+  if (readline === undefined) readline = require('readline');
   readline.moveCursor(this, dx, dy);
 };
 WriteStream.prototype.clearLine = function(dir) {
+  if (readline === undefined) readline = require('readline');
   readline.clearLine(this, dir);
 };
 WriteStream.prototype.clearScreenDown = function() {
+  if (readline === undefined) readline = require('readline');
   readline.clearScreenDown(this);
 };
 WriteStream.prototype.getWindowSize = function() {