Migrate flutter_tools file_system to null safety (#78896)

packages/flutter_tools/lib/src/base/file_system.dart

diff --git a/packages/flutter_tools/lib/src/base/file_system.dart b/packages/flutter_tools/lib/src/base/file_system.dart
index 38b8871..6cf75ad 100644
--- a/packages/flutter_tools/lib/src/base/file_system.dart
+++ b/packages/flutter_tools/lib/src/base/file_system.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 'package:file/file.dart';
 import 'package:file/local.dart' as local_fs;
 import 'package:meta/meta.dart';
@@ -31,8 +29,8 @@
 /// Various convenience file system methods.
 class FileSystemUtils {
   FileSystemUtils({
-    @required FileSystem fileSystem,
-    @required Platform platform,
+    required FileSystem fileSystem,
+    required Platform platform,
   }) : _fileSystem = fileSystem,
        _platform = platform;
 
@@ -86,8 +84,8 @@
   ///
   /// Returns false, if [entity] exists, but [referenceFile] does not.
   bool isOlderThanReference({
-    @required FileSystemEntity entity,
-    @required File referenceFile,
+    required FileSystemEntity entity,
+    required File referenceFile,
   }) {
     if (!entity.existsSync()) {
       return true;
@@ -97,8 +95,8 @@
   }
 
   /// Return the absolute path of the user's home directory.
-  String get homeDirPath {
-    String path = _platform.isWindows
+  String? get homeDirPath {
+    String? path = _platform.isWindows
       ? _platform.environment['USERPROFILE']
       : _platform.environment['HOME'];
     if (path != null) {
@@ -123,8 +121,8 @@
 void copyDirectory(
   Directory srcDir,
   Directory destDir, {
-  bool Function(File srcFile, File destFile) shouldCopyFile,
-  void Function(File srcFile, File destFile) onFileCopied,
+  bool Function(File srcFile, File destFile)? shouldCopyFile,
+  void Function(File srcFile, File destFile)? onFileCopied,
 }) {
   if (!srcDir.existsSync()) {
     throw Exception('Source directory "${srcDir.path}" does not exist, nothing to copy');
@@ -167,13 +165,13 @@
 
   @visibleForTesting
   LocalFileSystem.test({
-    @required Signals signals,
+    required Signals signals,
     List<ProcessSignal> fatalSignals = Signals.defaultExitSignals,
   }) : this(signals, fatalSignals, null);
 
-  Directory _systemTemp;
+  Directory? _systemTemp;
   final Map<ProcessSignal, Object> _signalTokens = <ProcessSignal, Object>{};
-  final ShutdownHooks _shutdownHooks;
+  final ShutdownHooks? _shutdownHooks;
 
   Future<void> dispose() async {
     _tryToDeleteTemp();
@@ -189,7 +187,7 @@
   void _tryToDeleteTemp() {
     try {
       if (_systemTemp?.existsSync() ?? false) {
-        _systemTemp.deleteSync(recursive: true);
+        _systemTemp?.deleteSync(recursive: true);
       }
     } on FileSystemException {
       // ignore.
@@ -225,6 +223,6 @@
         _tryToDeleteTemp,
       );
     }
-    return _systemTemp;
+    return _systemTemp!;
   }
 }