As something of a side note on this post, suggesting there is a single 'Dart/CoffeeScript route' that languages can take seems a bit misleading.
Dart is a whole different language that happens to be transpilable to JS. CoffeeScript is just a dash of syntactic sugar that (IMHO) makes JS a whole lot less tedious to write ('The golden rule of CoffeeScript is: "It's just JavaScript"').
Notwithstanding that, I am keen to try using TypeScript on a new project one of these days.
Dart is a whole different language that happens to be transpilable to JS. CoffeeScript is just a dash of syntactic sugar that (IMHO) makes JS a whole lot less tedious to write ('The golden rule of CoffeeScript is: "It's just JavaScript"').
Notwithstanding that, I am keen to try using TypeScript on a new project one of these days.