Update flutter dependencies in /.pub-cache
flutter pub cache repair or delete /Users/xxxxxxx/development/tools/flutter/.pub-cache/hosted/pub.dartlang.org/cloud_firestore-0.8.2+3/ and run flutter packages get again.
if all the above things fail delete the cache folder or also check the version updated in the lock file (some time lock give me the problem with updating the version number)
Disclaimer: By running the command above, have a really fast internetconnection or be ready to lose one hour of productive hours. ( it willredownload every package every installed on your pc, and I mean eachand all of the versions of each packages)~TSR
PS:Why flutter pub cache repair download every package version previously used?
Remove the package from dependencies in pubspec.yaml, run flutter packages get. And then add the package to dependencies again and running flutter packages get.This process has solved the problem for me in the past.
This steps worked for me
Delete the pubspec.lock file then run the command flutter pub get again.