fix args type in main method
fix args type in main method

Review URL: https://codereview.chromium.org/984433002

google/reflectable.dart,luisvt/reflectable,google/reflectable.dart,jakemac53/reflectable,bwu-dart-contributing/reflectable,jakemac53/reflectable,bwu-dart-contributing/reflectable,luisvt/reflectable