That's a fairly old article on Dart and doesn't really cover Dart's current direction (or even their direction back then).

The Dart team is very aware that it is highly unlikely that the DartVM will be put into any browser except Chrome (and there is no time-table on when that will happen). That is why their Dart2js compiler is seeing so much attention from the team.

Dart, to me, is Google's new GWT, but trying to align more with the web as we know it today (instead of hiding the DOM from its users). Even if Dart is only ever cross-compiled to Javascript and used that way in production, I would prefer to code in Dart than Javascript for anything that requires more than a few lines of javascript to implement.