[tools] migrate the rest of tools/ to null safety

Change-Id: Ieec55a99e9020f8f3962654e07518726d9f66fc4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/249540
Commit-Queue: Devon Carew <devoncarew@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

tools/test.dart

@@ -3,8 +3,6 @@
 // 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 = 2.9
-
 import 'package:test_runner/test_runner.dart';
 
 void main(List<String> args) {