fixup: test w/ callback

Author: ronag

benchmark/streams/writable-manywrites.js

@@ -6,10 +6,13 @@ const Writable = require('stream').Writable;
 const bench = common.createBenchmark(main, {
   n: [2e6],
   sync: ['yes', 'no'],
-  writev: ['yes', 'no']
+  writev: ['yes', 'no'],
+  callback: ['yes', 'no']
 });
 
-function main({ n, sync, writev }) {
+function nop() {}
+
+function main({ n, sync, writev, callback }) {
   const b = Buffer.allocUnsafe(1024);
   const s = new Writable();
   sync = sync === 'yes';
@@ -30,11 +33,13 @@ function main({ n, sync, writev }) {
     };
   }
 
+  const cb = callback === 'yes' ? nop : null;
+
   bench.start();
 
   let k = 0;
   function run() {
-    while (k++ < n && s.write(b));
+    while (k++ < n && s.write(b, cb));
     if (k >= n)
       bench.end(n);
   }