exported jsoN_keys.dart and property.dart

lib/s3i_flutter.dart

@@ -24,6 +24,7 @@ export 'src/directory/endpoint.dart';
 export 'src/directory/event.dart';
 export 'src/directory/link.dart';
 export 'src/directory/location.dart';
+export 'src/directory/property.dart';
 export 'src/directory/service.dart';
 export 'src/directory/thing.dart';
 export 'src/directory/value.dart';
@@ -49,3 +50,5 @@ export 'src/query/query_assembler.dart';
 export 'src/query/rql_query.dart';
 //core
 export 'src/s3i_core.dart';
+//util
+export 'src/utils/json_keys.dart';