think of all the AS3 developers starting to feel left out in the cold now that Adobe's walking away from Flash.
while i'd agree that now's probably the time to jump into JS and HTML5, some people think Dart has a future, and I guess this opens up its potential market.
Adobe is walking away from the mobile player but still has Air out in the wild wich produces a lot of good chart topping titles on the Android and iOS app stores.
In my opinion, it is better to pick up Haxe if someone is coming from AS3. You can use javascript for the HTML5 compile target, you get extended Mr. Doob three.js support provided by the author himself, node.js through the Noxe Lib extends, and much more. And that is just for the HTML5 target.
It even target compiles to Android and iOS through the Haxe NME framework.
They are adding Java support to the thing and it has been doing c++ and php for some years. There are no silly ";" fights and honestly it is good to be able to use the same code source to run on multiple plats.
Though Dart may well pick up and Easel is already becoming a thing.
This might be useful as stop-gap. But I would hate to bug-test anything built using this method.