use symbol for storing destroyingOnFinish flag

Author: arturgawlik

lib/net.js

@@ -152,6 +152,7 @@ const DEFAULT_IPV6_ADDR = '::';
 const noop = () => {};
 
 const kPerfHooksNetConnectContext = Symbol('kPerfHooksNetConnectContext');
+const kDestroyingOnFinish = Symbol('kDestroyingOnFinish');
 
 const dc = require('diagnostics_channel');
 const netClientSocketChannel = dc.channel('net.client.socket');
@@ -795,17 +796,17 @@ function onReadableStreamEnd() {
 
 
 Socket.prototype.destroySoon = function() {
-  if (this.destroyingOnFinish) return;
+  if (this[kDestroyingOnFinish]) return;
 
   if (this.writable)
     this.end();
 
   if (this.writableFinished)
     this.destroy();
   else {
-    this.destroyingOnFinish = true;
+    this[kDestroyingOnFinish] = true;
     this.once('finish', () => {
-      this.destroyingOnFinish = false;
+      this[kDestroyingOnFinish] = false;
       this.destroy();
     });
   }