flutter run failed with error "Could not resolve the package 'characters' in 'package:characters/characters.dart'."

I faced this error too while updating Flutter today to Flutter Channel beta, 1.20.0-7.2.pre for an existing project. As mentioned in https://github.com/flutter/flutter/issues/59645#issuecomment-645236663 running: flutter clean solved the problem. This actually deletes the .dart_tool plus a few other files.

I solved the the issue installing https://pub.dev/packages/characters

I faced with this Error today (when i updated to Flutter 1.20.0-7.2.pre • channel beta) and finaly i solved it with deleting the .dart_tool (location: ../yourProject/.dart_tool) folder in my project and run my project again.