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 language on it.
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.
Check tag count for questions on SO; 4500 for CoffeeScript, 1500 for Dart.
Not to mention the violent revulsion many people (myself included) are expressing in this topic.
On that note, Java has 508,338 questions on StackOverflow, which crushes CoffeeScript, /especially/ with those 15,000 GWT questions ;)
As someone else pointed out, SO isn't a great metric, but finding github activity by language seemed to take more time than I was willing to invest.
Immaturity seems like a good explanation of that effect
:P