Flutter autocomplete not working in VSCode for packages
You must make sure that your dependencies for plugins are mentioned under dependencies: section, not under dev_dependencies: section like following:
dependencies: flutter: sdk: flutter cloud_firestore: ^0.13.7 firebase_auth: ^0.16.1 fluttertoast: ^7.0.2
I was getting the same issue and fixed it by changing the flutter channel.The problem seems to be with the dart that ships with the flutter code.
If you are still facing this issue. Then try to change your channel
flutter channel ...
And then try
flutter upgrade
This will pull the dart version again and hopefully fix the import issue.