Migrate dwds/tool directory to null safety (#1647)

* Migrate dwds/tool directory to null safety

* Ignored false positive analyzer warning

* Addressed PR comments

Author: Anna Gringauze

dwds/tool/copy_builder.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 = 2.9
-
 import 'package:build/build.dart';
 
 /// Factory for the build script.