Add dart language comment "// @Dart=2.10" to top of dart files.

main.dart

// @dart = 2.10
import 'dart:io';
import 'package:glob/glob.dart';

Future<void> main() async {
  final replacements =
      Glob('**.dart').list().asyncMap((fileSystemEntity) async {
    if (fileSystemEntity is File) {
      final content = await fileSystemEntity.readAsString();
      await fileSystemEntity.writeAsString('// @dart = 2.10\n$content');
    }
  });
  await replacements.last;
}