When creating a release, pub_release will create or update a dart file containing the package version no.
The version file is called:
lib/src/version/version.g.dart
The contents of the version file will be:
/// GENERATED BY pub_release do not modify.
/// pub_release version
String packageVersion = '3.0.0';
Where the packageVersion is set to the value in your pubspec.yaml.
The packageVersion variable provides a convenient method if you need to display the version no. of your package.