Fix warning with Dart 2.19 beta
Fix warning with Dart 2.19 beta

google/protobuf.dart,google/protobuf.dart,dart-lang/protobuf,dart-lang/dart-protobuf,dart-lang/dart-protobuf,dart-lang/protobuf,dart-lang/dart-protobuf,dart-lang/protobuf,google/protobuf.dart