Annotations need to be imported
Adding at the top of your file
import 'package:flutter/foundation.dart';
should fix it.
Annotations the DartAnalyzer understands are provided by the meta package.
To make it easier for Flutter developers, the Flutter team decided to add the meta package to the Flutter SDK and re-export it in flutter/foundation.dart. The annotations by flutter are therefore exactly the same as these provided by the meta package and you can as well add meta to your dependencies in pubspec.yaml and import annotations from there if you prefer. If you want to reuse code between for example AngularDart and Flutter that is the preferred way because code that imports from package:flutter/... can’t be used in Dart web applications.