Most of the work is done completely in the open. See some of the language discussion writeups: https://groups.google.com/a/dartlang.org/forum/#!searchin/mi...
We went public two years ago when the language was still in its infancy so that developers could provide their feedback.
Edit: contrary to your claim we did get feedback and acted on it. Some changes were rejected (like putting the types behind the variables), but it's normal that not every request can be accepted.
If you have suggestions on how we can make Dart even more open-source I would be genuinely interested in hearing them.