pubspec.yaml

# ...
dependencies:
  flutter:
    sdk: flutter
  cupertino_icons: ^1.0.2
  firebase_core: ^1.2.0
  firebase_auth: ^1.2.0
  firebase_storage: ^8.1.0
  file_picker: ^3.0.2+2
  cloud_firestore: ^2.2.0 # この行を追加