Revert "[minor] Set the flag to detect invalid API usage only when ne…

…eded"

This reverts commit 91b5173.

lib/websocket-server.js

@@ -79,17 +79,18 @@ class WebSocketServer extends EventEmitter {
         callback
       );
     } else if (options.server) {
-      if (options.server[kUsedByWebSocketServer]) {
+      this._server = options.server;
+    }
+
+    if (this._server) {
+      if (this._server[kUsedByWebSocketServer]) {
         throw new Error(
           'The HTTP/S server is already being used by another WebSocket server'
         );
+      } else {
+        this._server[kUsedByWebSocketServer] = true;
       }
 
-      options.server[kUsedByWebSocketServer] = true;
-      this._server = options.server;
-    }
-
-    if (this._server) {
       this._removeListeners = addListeners(this._server, {
         listening: this.emit.bind(this, 'listening'),
         error: this.emit.bind(this, 'error'),
@@ -143,6 +144,8 @@ class WebSocketServer extends EventEmitter {
     const server = this._server;
 
     if (server) {
+      server[kUsedByWebSocketServer] = false;
+
       this._removeListeners();
       this._removeListeners = this._server = null;
 
@@ -153,8 +156,6 @@ class WebSocketServer extends EventEmitter {
         server.close(() => this.emit('close'));
         return;
       }
-
-      delete server[kUsedByWebSocketServer];
     }
 
     process.nextTick(emitClose, this);