enable dangling_library_doc_comments (dart-archive/linter#3980)

Author: pq

analysis_options.yaml

@@ -39,6 +39,7 @@ linter:
 #    - combinators_ordering
 #    - close_sinks # https://github.com/dart-lang/linter/issues/268
     - comment_references
+    - dangling_library_doc_comments
     - directives_ordering
     - discarded_futures
     - join_return_with_assignment

lib/src/ast.dart

@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 /// Common AST helpers.
+library;
+
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/syntactic_entity.dart';
 import 'package:analyzer/dart/ast/token.dart';

pubspec.yaml

@@ -9,7 +9,7 @@ repository: https://github.com/dart-lang/linter
 documentation: https://dart-lang.github.io/linter/lints
 
 environment:
-  sdk: '>=2.17.0 <3.0.0'
+  sdk: '>=2.19.0 <3.0.0'
 
 dependencies:
   analyzer: ^5.4.0