streams2: Fix regression from Duplex ctor assignment

lib/_stream_writable.js

@@ -27,7 +27,6 @@ module.exports = Writable
 
 var util = require('util');
 var Stream = require('stream');
-var Duplex = require('_stream_duplex');
 
 util.inherits(Writable, Stream);
 
@@ -60,7 +59,7 @@ function WritableState(options) {
 function Writable(options) {
   // Writable ctor is applied to Duplexes, though they're not
   // instanceof Writable, they're instanceof Readable.
-  if (!(this instanceof Writable) && !(this instanceof Duplex))
+  if (!(this instanceof Writable) && !(this instanceof Stream.Duplex))
     return new Writable(options);
 
   this._writableState = new WritableState(options);