https://medium.com/flutter/testable-flutter-and-cloud-firestore-1cf2fbbce97b >
This article assumes that you have watched and followed the steps in this tutorial to integrate your app with Firebase. To recap:
Create a new Flutter project, and call it firebasecounter.
Create a Firebase app in the Firebase console.
Link your app to iOS and/or Android, depending on your development environment and target audience.
Note: If you configure your app to work on an Android client, make sure that you create a debug.keystore file before generating your SHA1 certificate.
After you generate your iOS or Android apps in Firebase, you are ready to proceed. The rest of the video contains great content that you will likely need for real projects, but it’s not required for this tutorial.