[dart2js] Migrate serialization/strategies.dart to null safety.

Change-Id: Iad23474f69b78231a6c77da5f5b8b379618f77be
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/267820
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Nate Biggs <natebiggs@google.com>
diff --git a/pkg/compiler/lib/src/serialization/strategies.dart b/pkg/compiler/lib/src/serialization/strategies.dart
index 86ba9cc..fc3e786 100644
--- a/pkg/compiler/lib/src/serialization/strategies.dart
+++ b/pkg/compiler/lib/src/serialization/strategies.dart
@@ -2,8 +2,6 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// @dart = 2.10
-
 import 'dart:io';
 
 import 'package:front_end/src/fasta/kernel/utils.dart' as ir