Add missing license header.

Author: ditman

packages/cross_file/lib/src/legacy/legacy_factory.dart

@@ -1,2 +1,6 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
 export 'legacy_factory_io.dart'
     if (dart.library.js_interop) 'legacy_factory_web.dart';