Many other languages and projects use underscores to indicate private properties, in particular Google's Closure. I also have no idea how standardizing on _ in names meaning private would do anything to push Dart in some hidden, evil agenda.

I think the change is reasonable, limiting AngularJS' expression to leak less information into the templates is a good idea IMHO, as described in particular with the new controller syntax.

Disclosure: I work at Google, but I have not been involved in this discussion at all.