stream: use plain objects for write/corked reqs

This is similar to a change made awhile back for storing
process.nextTick() requests.

PR-URL: https://github.com/nodejs/node/pull/10558
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

lib/_stream_writable.js

@@ -16,13 +16,6 @@ util.inherits(Writable, Stream);
 
 function nop() {}
 
-function WriteReq(chunk, encoding, cb) {
-  this.chunk = chunk;
-  this.encoding = encoding;
-  this.callback = cb;
-  this.next = null;
-}
-
 function WritableState(options, stream) {
   options = options || {};
 
@@ -113,7 +106,9 @@ function WritableState(options, stream) {
 
   // allocate the first CorkedRequest, there is always
   // one allocated and free to use, and we maintain at most two
-  this.corkedRequestsFree = new CorkedRequest(this);
+  var corkReq = { next: null, entry: null, finish: undefined };
+  corkReq.finish = onCorkedFinish.bind(undefined, corkReq, this);
+  this.corkedRequestsFree = corkReq;
 }
 
 WritableState.prototype.getBuffer = function getBuffer() {
@@ -304,7 +299,7 @@ function writeOrBuffer(stream, state, isBuf, chunk, encoding, cb) {
 
   if (state.writing || state.corked) {
     var last = state.lastBufferedRequest;
-    state.lastBufferedRequest = new WriteReq(chunk, encoding, cb);
+    state.lastBufferedRequest = { chunk, encoding, callback: cb, next: null };
     if (last) {
       last.next = state.lastBufferedRequest;
     } else {
@@ -423,7 +418,9 @@ function clearBuffer(stream, state) {
       state.corkedRequestsFree = holder.next;
       holder.next = null;
     } else {
-      state.corkedRequestsFree = new CorkedRequest(state);
+      var corkReq = { next: null, entry: null, finish: undefined };
+      corkReq.finish = onCorkedFinish.bind(undefined, corkReq, state);
+      state.corkedRequestsFree = corkReq;
     }
   } else {
     // Slow case, write chunks one-by-one
@@ -528,14 +525,6 @@ function endWritable(stream, state, cb) {
   stream.writable = false;
 }
 
-// It seems a linked list but it is not
-// there will be only 2 of these for each stream
-function CorkedRequest(state) {
-  this.next = null;
-  this.entry = null;
-  this.finish = onCorkedFinish.bind(undefined, this, state);
-}
-
 function onCorkedFinish(corkReq, state, err) {
   var entry = corkReq.entry;
   corkReq.entry = null;