library_annotations 
            
            Attach library annotations to library directives.
This rule is available as of Dart 2.19.0.
This rule has a quick fix available.
Details
Attach library annotations to library directives, rather than some other library-level element.
BAD:
@TestOn('browser')
import 'package:test/test.dart';
void main() {}
GOOD:
@TestOn('browser')
library;
import 'package:test/test.dart';
void main() {}
NOTE: An unnamed library, like library; above, is only supported in Dart
2.19 and later. Code which might run in earlier versions of Dart will need to
provide a name in the library directive.
Usage
To enable the library_annotations rule,
add library_annotations under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - library_annotations