Migrate some tool commands to NNBD (#3899)

Now that individual commands can be migrated, migrate several commands
that are trivially migratable.

Part of https://github.com/flutter/flutter/issues/81912

script/tool/lib/src/format_command.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=2.9
-
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io' as io;