Tried outside of company network, it worked. Sorry, it was caused by company's proxy server, could not access dartservices:
POST
https://dart-services.appspot.com/api/dartservices/v1/analyze?alt=json 403 (Forbidden)dart.oe.$1 @ main.dart.js:11862dart.zZ.eK @ main.dart.js:4927dart.z8.$0 @ main.dart.js:
3931dart.A.static.bN @ main.dart.js:3885dart.A.d_ @ main.dart.js:3808dart.z3.$0 @ main.dart.js:3924dart.ln.ji @ main.dart.js:4027dart.AR @ main.dart.js:3391dart.F7 @ main.dart.js:3393dart.yj.$1 @ main.dart.js:3502dart.BR.$0 @ main.dart.js:
2086dart.hr.dj @ main.dart.js:956dart.dw @ main.dart.js:756dart.BP @ main.dart.js:1546(anonymous function) @ main.dart.js:1555