Migrate dartdoc to null safety (#84153)

dev/tools/dartdoc.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:convert';
 import 'dart:io';
 
@@ -258,9 +256,9 @@ String getBranchName() {
   final ProcessResult gitResult = Process.runSync('git', <String>['status', '-b', '--porcelain']);
   if (gitResult.exitCode != 0)
     throw 'git status exit with non-zero exit code: ${gitResult.exitCode}';
-  final Match gitBranchMatch = gitBranchRegexp.firstMatch(
+  final RegExpMatch? gitBranchMatch = gitBranchRegexp.firstMatch(
       (gitResult.stdout as String).trim().split('\n').first);
-  return gitBranchMatch == null ? '' : gitBranchMatch.group(1).split('...').first;
+  return gitBranchMatch == null ? '' : gitBranchMatch.group(1)!.split('...').first;
 }
 
 String gitRevision() {
@@ -314,7 +312,7 @@ void createSearchMetadata(String templatePath, String metadataPath) {
 /// specified, for each source/destination file pair.
 ///
 /// Creates `destDir` if needed.
-void copyDirectorySync(Directory srcDir, Directory destDir, [void Function(File srcFile, File destFile) onFileCopied]) {
+void copyDirectorySync(Directory srcDir, Directory destDir, [void Function(File srcFile, File destFile)? onFileCopied]) {
   if (!srcDir.existsSync())
     throw Exception('Source directory "${srcDir.path}" does not exist, nothing to copy');
 

dev/tools/dartdoc_checker.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// @dart = 2.8
-
 import 'dart:io';
 
 import 'package:path/path.dart' as path;
@@ -82,7 +80,7 @@ void checkForUnresolvedDirectives(String htmlOutputPath) {
 int _scanFile(File file) {
   assert(path.extension(file.path) == '.html');
   final Iterable<String> matches = _pattern.allMatches(file.readAsStringSync())
-      .map((RegExpMatch m ) => m.group(0));
+      .map((RegExpMatch m ) => m.group(0)!);
 
   if (matches.isNotEmpty) {
     stderr.writeln('Found unresolved dartdoc directives in ${file.path}:');