Add missing copyright

pkgs/yaml/lib/src/error_listener.dart

diff --git a/pkgs/yaml/lib/src/error_listener.dart b/pkgs/yaml/lib/src/error_listener.dart
index c302a4d..dac7d63 100644
--- a/pkgs/yaml/lib/src/error_listener.dart
+++ b/pkgs/yaml/lib/src/error_listener.dart
@@ -1,3 +1,7 @@
+// Copyright (c) 2021, 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 'yaml_exception.dart';
 
 /// A listener that is notified of [YamlError]s during scanning/parsing.