Mainstream is not a "moving goalpost fallacy". It was stated from the starting comment that he was talking about a "mainstream" language.
Of course the "mainstreamity" of languages is hard to determine, but Dart is so low in any kind of related metric that makes this a non-issue.
Let's put it this way: the Dart team announced a few months ago that they'd be concentrating on transpiling [1] and they won't put the VM officially in Chrome, and nobody gave much of a fuck judging from reactions in blogs, forums, HN etc.
If a mainstream language changed something, even something minor, there would have been blood (as it often happens).