fix warnings

cherrypick_flutter/lib/cherrypick_flutter.dart

@@ -1,3 +1,5 @@
+library;
+
 ///
 /// Copyright 2021 Sergey Penkovsky <sergey.penkovsky@gmail.com>
 /// Licensed under the Apache License, Version 2.0 (the "License");
@@ -10,6 +12,5 @@
 /// See the License for the specific language governing permissions and
 /// limitations under the License.
 ///
-library cherrypick_flutter;
 
 export 'src/cherrypick_provider.dart';