Careers in software are (hopefully) long and varied.
Committing to work for a few years on one particular technology should not make or break your career.
As long as we're always learning and sharing our knowledge and skills, we should all be better off.
Regardless of whether Dart ever hits the mainstream.
Yes, the author is probably a bit junior in that field. You don't bet your career on technologies, much less languages (especially beleaguered languages like Dart).
You take a bet, see if it works for a few years and if it doesn't (which is the case for 99% of languages out there), you move on to something else.
At least, that's been my experience as an average/middling dev - there may well be some level of zen where this doesn't apply, but I'm hard pressed to think of one right now.