fix(page): page.close() assert that connection is not closed (#1038)

lib/Connection.js

@@ -201,6 +201,7 @@ class Session extends EventEmitter {
   }
 
   async dispose() {
+    console.assert(!!this._connection, 'Protocol error: Connection closed. Most likely the page has been closed.');
     await this._connection.send('Target.closeTarget', {targetId: this._targetId});
   }