build: use online: false for browsersync (#18114)

Author: jelbourn

tools/dev-server/dev-server.ts

@@ -23,6 +23,7 @@ export class DevServer {
   /** Options of the browser-sync server. */
   options: browserSync.Options = {
     open: false,
+    online: false,
     port: this.port,
     notify: false,
     ghostMode: false,