stream: use standard for loop instead of for..of

Use the standard `for` loop style instead of `for..of` for
consistency.

PR-URL: nodejs#41871
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Minwoo Jung <nodecorelab@gmail.com>

lib/internal/streams/duplex.js

@@ -42,8 +42,10 @@ ObjectSetPrototypeOf(Duplex.prototype, Readable.prototype);
 ObjectSetPrototypeOf(Duplex, Readable);
 
 {
+  const keys = ObjectKeys(Writable.prototype);
   // Allow the keys array to be GC'ed.
-  for (const method of ObjectKeys(Writable.prototype)) {
+  for (let i = 0; i < keys.length; i++) {
+    const method = keys[i];
     if (!Duplex.prototype[method])
       Duplex.prototype[method] = Writable.prototype[method];
   }