[tools] verify_docs don't error on library annotations

Dartdoc code snippets should be able to have annotations before the
`library` keyword.

Bug: #49803 (comment)

Change-Id: I7ec0b6db296274a1173d10d900f7af38a1f2b552
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/265321
Reviewed-by: Devon Carew <[email protected]>
Auto-Submit: Daco Harkes <[email protected]>
Commit-Queue: Daco Harkes <[email protected]>
Commit-Queue: Devon Carew <[email protected]>

Author: dcharkes

tools/verify_docs/bin/verify_docs.dart

@@ -18,6 +18,7 @@ import 'package:analyzer/error/error.dart';
 import 'package:analyzer/file_system/overlay_file_system.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/source/line_info.dart';
+import 'package:analyzer/src/dart/error/syntactic_errors.g.dart';
 import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer/src/util/comment.dart';
 import 'package:path/path.dart' as path;
@@ -353,6 +354,10 @@ class ValidateCommentCodeSamplesVisitor extends GeneralizingAstVisitor {
         return false;
       });
 
+      // Don't error on annotations above library.
+      errors.removeWhere(
+          (e) => e.errorCode == ParserErrorCode.LIBRARY_DIRECTIVE_NOT_FIRST);
+
       if (errors.isNotEmpty) {
         print('$filePath:${sample.lineStartOffset}: ${errors.length} errors');