Add missing license header to license.dart (#166)

diff --git a/pkgs/firehose/lib/src/health/license.dart b/pkgs/firehose/lib/src/health/license.dart
index ffc97b0..0a2a657 100644
--- a/pkgs/firehose/lib/src/health/license.dart
+++ b/pkgs/firehose/lib/src/health/license.dart
@@ -1,3 +1,7 @@
+// Copyright (c) 2023, 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:io';
 
 import 'package:path/path.dart' as path;