[flutter] Error: The class ‘FileSystemEvent’ can’t be extended, implemented, or mixed in outside of its library because it’s a sealed class. の対処メモ

flutterでアプリ開発してたらエラーではまったのでその対応メモです。

なお開発環境は次の通りです。

Windows 10
flutter 3.22.0

事象
原因
対応方法

事象

mockを更新しようと次のコマンドを実行すると

$ dart run build_runner build

以下のようなエラーが出て失敗しました。

Resolving dependencies in `E:\sample_project`...
Downloading packages...
Got dependencies in `E:\sample_project`.
Building package executable... (1.0s)
Failed to build build_runner:build_runner:
/C:/Users/User/AppData/Local/Pub/Cache/hosted/pub.dev/watcher-1.0.2/lib/src/constructable_file_system_event.dart:7:57: Error: The class 'FileSystemEvent' can't be extended, implemented, or mixed in outside of its library because it's a sealed class.
abstract class _ConstructableFileSystemEvent implements FileSystemEvent {
                                                        ^
Failed to build build_runner:build_runner:
/C:/Users/User/AppData/Local/Pub/Cache/hosted/pub.dev/watcher-1.0.2/lib/src/constructable_file_system_event.dart:7:57: Error: The class 'FileSystemEvent' can't be extended, implemented, or mixed in outside of its library because it's a sealed class.
abstract class _ConstructableFileSystemEvent implements FileSystemEvent {