I was going to object, but in doing my own research I found that Haskell ranked even below Dart in the SO Developer Survey for languages in use - and I certainly wouldn't consider Dart popular, since I've never seen, let alone met, anyone using it.
That said, "popular" is a very contextual term. Within certain classes of programmers (and I don't mean the obvious tautological one) Haskell is extremely popular, and fit for purpose. I assume the same is true for Dart.
And I would add that, in those circles, the introduction of advanced HKT features like RankNTypes has been extremely successful.
template template parameters and nested template classes work as HKT in C++ in practice (and are extensively used for exactly that purpose), and I think C++ counts as a popular language.
I was surprised that to learn that HKT weren't part of Scala from the beginning and had to look that up. You're a bit wrong about when that happened: it was Scala 2.5 in 2007, not 2.8 which was released in 2010.
