IE8 usage is under 8% so it's possible for some people/organizations to stop supporting it. In another year, I would expect market share to be under 5%.
There can be many markets where IE usage is much higher than the worldwide average. My previous job had IE as a whole with ~60% share. You can't just decide to ditch a browser because of averages, you need to look at your own data.
Corporate Intranet: within epsilon of 100% IE 8, at the moment. My current job and the last one always had some outdated version of Internet Exploder as the desktop standard (which I ignored for running anything other than in-house junk -- no way I'm taking MSIE out into the wild)
It's not reasonable to support an outdated browser version with an inferior Javascript engine, or to single out Internet Explorer as a browser for which older versions should still be supported, imho.
(disclaimer: I'm building a webapp that has to work in IE 8)
I wouldn't mind if Dart didn't support older versions of Chrome or Firefox because that's just much different than IE 8.
Most people who are running IE 8 are random non-technical folks who are using the browser that came with their OS. They don't even know what "internet explorer" is.
There are probably a small sub-set of people who have Firefox 3.x because their brother's uncle's nephew installed it once like 5 years ago but I honestly don't mind losing these people as potential customers because it's such a ridiculously small %. I also feel like these are the type of people who would be more than capable of upgrading if they kept seeing messages like "your browser is old as time itself, upgrade or find someone who knows how to do it for you!" through friend/family assistance.
I can't justify throwing away almost 10% (IE 8) market share for Dart and the idea of supporting multiple versions of the app is just too much work for too little return.
It's not reasonable to throw away all IE 8 traffic just to get Dart's features.