My comment was in reply to someone claiming that Dart was "yet-another Proprietary Google Technology". If Dart is proprietary then Qt is, too. Most likely by proprietary they didn't mean license but being controlled by a company, which both are. Dart is open source btw. as well. In fact, you could say that Dart is more open source because in Qt, proprietary ISV developers can't use the open source model if they want to not avoid having to give dynamic linking support and other LGPL obligations.

And trolltech is very alive and publicly traded in fact. It has just been renamed to Qt company: https://en.wikipedia.org/wiki/The_Qt_Company

Dart and Flutter are both BSD licensed, so presumably the discussion of "proprietary" is at least somewhat orthogonal to whether it has an open source license.