stream: Duplex re-use Writable properties

Instead of reimplementing Writable properties, fetch them
from the Writable prototype.

lib/_stream_duplex.js

@@ -71,7 +71,20 @@ function Duplex(options) {
 }
 
 ObjectDefineProperties(Duplex.prototype, {
-  writable: ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writable'),
+  writable:
+    ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writable'),
+  writableHighWaterMark:
+    ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writableHighWaterMark'),
+  writableBuffer:
+    ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writableBuffer'),
+  writableLength:
+    ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writableLength'),
+  writableFinished:
+    ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writableFinished'),
+  writableCorked:
+    ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writableCorked'),
+  writableEnded:
+    ObjectGetOwnPropertyDescriptor(Writable.prototype, 'writableEnded'),
 
   destroyed: {
     get() {
@@ -89,41 +102,5 @@ ObjectDefineProperties(Duplex.prototype, {
         this._writableState.destroyed = value;
       }
     }
-  },
-
-  writableHighWaterMark: {
-    get() {
-      return this._writableState && this._writableState.highWaterMark;
-    }
-  },
-
-  writableBuffer: {
-    get() {
-      return this._writableState && this._writableState.getBuffer();
-    }
-  },
-
-  writableLength: {
-    get() {
-      return this._writableState && this._writableState.length;
-    }
-  },
-
-  writableFinished: {
-    get() {
-      return this._writableState ? this._writableState.finished : false;
-    }
-  },
-
-  writableCorked: {
-    get() {
-      return this._writableState ? this._writableState.corked : 0;
-    }
-  },
-
-  writableEnded: {
-    get() {
-      return this._writableState ? this._writableState.ending : false;
-    }
   }
 });