zlib: replace usage of internal stream state with public api

Refs: #445

PR-URL: #34884
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

lib/zlib.js

@@ -355,17 +355,15 @@ const kFlushBuffers = [];
 }
 
 ZlibBase.prototype.flush = function(kind, callback) {
-  const ws = this._writableState;
-
   if (typeof kind === 'function' || (kind === undefined && !callback)) {
     callback = kind;
     kind = this._defaultFullFlushFlag;
   }
 
-  if (ws.ended) {
+  if (this.writableFinished) {
     if (callback)
       process.nextTick(callback);
-  } else if (ws.ending) {
+  } else if (this.writableEnded) {
     if (callback)
       this.once('end', callback);
   } else {
@@ -392,8 +390,7 @@ ZlibBase.prototype._transform = function(chunk, encoding, cb) {
   }
 
   // For the last chunk, also apply `_finishFlushFlag`.
-  const ws = this._writableState;
-  if ((ws.ending || ws.ended) && ws.length === chunk.byteLength) {
+  if (this.writableEnded && this.writableLength === chunk.byteLength) {
     flushFlag = maxFlush(flushFlag, this._finishFlushFlag);
   }
   processChunk(this, chunk, flushFlag, cb);