dart: Replace stagehand with dcli or commit pubspec.yaml
dcli looks like a better fit for the moment than stagehand, esp if we can get sprintf added to their default set.

But I ran into https://github.com/bsutton/dcli/issues/139

Alternatively we could commit pubspec.yaml (mentioned at https://github.com/adsharma/py2many/pull/112#pullrequestreview-647778212 ), or build our own project starter, ala https://github.com/adsharma/py2many/issues/158