Earlier quoted context omitted.
"I will never use Dart" That's quite a sure statement. Why don't you think Dart will ever be natively adopted by Firefox at some point in the future?
Because that's a pretty laughable idea. There's no reason for browsers to choose Dart over CoffeeScript or TypeScript, and Dart is so far behind CoffeeScript it's not even funny. Google wants to do it, sure, but that doesn't mean they'll succeed, even if they try something as stubborn as running Dart alongside Javascript in Chrome. There simply isn't a path to ubiquity for Dart, and if there were it's not the only la…
Dart is fundamentally different in that it was designed for speed (as well as toolability and readability) and by directly supporting Dart a VM can make many more optimizations than a JS VM can. The Dart VM is already 1.5-2x faster than JS in some cases, while being _much_ younger.