migrate host agent (#84755)

dev/devicelab/lib/framework/host_agent.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';
 import 'package:meta/meta.dart';
@@ -14,7 +12,7 @@ HostAgent get hostAgent => HostAgent(platform: const LocalPlatform(), fileSystem
 
 /// Host machine running the tests.
 class HostAgent {
-  HostAgent({@required Platform platform, @required FileSystem fileSystem})
+  HostAgent({required Platform platform, required FileSystem fileSystem})
       : _platform = platform,
         _fileSystem = fileSystem;
 
@@ -22,19 +20,19 @@ class HostAgent {
   final FileSystem _fileSystem;
 
   /// Creates a directory to dump file artifacts.
-  Directory/*?*/ get dumpDirectory {
+  Directory? get dumpDirectory {
     if (_dumpDirectory == null) {
       // Set in LUCI recipe.
-      final String directoryPath = _platform.environment['FLUTTER_LOGS_DIR'];
+      final String? directoryPath = _platform.environment['FLUTTER_LOGS_DIR'];
       if (directoryPath != null) {
         _dumpDirectory = _fileSystem.directory(directoryPath)..createSync(recursive: true);
-        print('Found FLUTTER_LOGS_DIR dump directory ${_dumpDirectory.path}');
+        print('Found FLUTTER_LOGS_DIR dump directory ${_dumpDirectory?.path}');
       }
     }
     return _dumpDirectory;
   }
 
-  static Directory/*?*/ _dumpDirectory;
+  static Directory? _dumpDirectory;
 
   @visibleForTesting
   void resetDumpDirectory() {