Why isn't JSON output ?

[MrLeoZou]

I execute from the command line: dart ./lib/cli/generator.dart -p samples/example.dart
But only the following is output，There is no JSON output. Why is that?
lib/cli/generator.dart: Warning: Interpreting this as package URI, 'package:flutter_ast/cli/generator.dart'.
100% [=================================================================================================================================================================================================================================================================>]-- RESULTS --
Classes: 2
Enums: 1
Imports: 1
Methods: 1
ast_generate.txt