In what way is Dart "so far behind" CoffeeScript?
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