add license

lib/src/identity_codec.dart

diff --git a/lib/src/identity_codec.dart b/lib/src/identity_codec.dart
index 2df6144..0a2a0b5 100644
--- a/lib/src/identity_codec.dart
+++ b/lib/src/identity_codec.dart
@@ -1,3 +1,7 @@
+// Copyright (c) 2022, 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';
 
 class _IdentityConverter<T> extends Converter<T, T> {