stream: Remove unnecessary nextTick usage in Writable

Fix #4928

lib/_stream_writable.js

@@ -240,31 +240,35 @@ function onwrite(stream, er) {
   if (er)
     onwriteError(stream, state, sync, er, cb);
   else {
-    if (!finishMaybe(stream, state)) {
-      if (state.length === 0 && state.needDrain)
-        onwriteDrain(stream, state);
+    var finished = finishMaybe(stream, state);
 
-      if (!state.bufferProcessing && state.buffer.length)
-        clearBuffer(stream, state);
-    }
+    if (!finished && !state.bufferProcessing && state.buffer.length)
+      clearBuffer(stream, state);
 
-    if (sync)
-      process.nextTick(cb);
-    else
-      cb();
+    if (sync) {
+      process.nextTick(function() {
+        afterWrite(stream, state, finished, cb);
+      });
+    } else {
+      afterWrite(stream, state, finished, cb);
+    }
   }
 }
 
+function afterWrite(stream, state, finished, cb) {
+  if (!finished)
+    onwriteDrain(stream, state);
+  cb();
+}
+
 // Must force callback to be called on nextTick, so that we don't
 // emit 'drain' before the write() consumer gets the 'false' return
 // value, and has a chance to attach a 'drain' listener.
 function onwriteDrain(stream, state) {
-  process.nextTick(function() {
-    if (state.needDrain) {
-      state.needDrain = false;
-      stream.emit('drain');
-    }
-  });
+  if (state.length === 0 && state.needDrain) {
+    state.needDrain = false;
+    stream.emit('drain');
+  }
 }