add license header

Author: lesnitsky

Diff for: packages/firebase_ui_shared/lib/src/universal_alert.dart

@@ -1,3 +1,7 @@
+// Copyright 2023, the Chromium 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 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';