anchor.diff:    @override
     void visitClassTypeAlias(ClassTypeAlias node) {
       checkComment(node.documentationComment);
  +  }
  +
  +  @override
  +  void visitCompilationUnit(CompilationUnit node) {
  +    var directives = node.directives;
  +    if (directives.isNotEmpty) {
  +      checkComment(directives[0].documentationComment);
  +    }
     }
   
     @override
  
positive.code:   }
  
    @override
    void visitLibraryDirective(LibraryDirective node) {
      checkComment(node.documentationComment);
    }
  
    @override
    void visitMethodDeclaration(MethodDeclaration node) {
      checkComment(node.documentationComment);
    }
  
negative.code:     final visitor = _Visitor(this);
      registry.addClassDeclaration(this, visitor);
      registry.addClassTypeAlias(this, visitor);
      registry.addCompilationUnit(this, visitor);
      registry.addConstructorDeclaration(this, visitor);
      registry.addEnumConstantDeclaration(this, visitor);
      registry.addEnumDeclaration(this, visitor);
  