Add license to file

Author: mosuem

pkgs/convert/test/identity_codec_test.dart

@@ -1,3 +1,7 @@
+// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
+// 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.
+
 import 'dart:convert';
 import 'package:convert/convert.dart';
 import 'package:test/test.dart';