I wouldn’t expect Dart to become a major language; it’s clearly tied to Flutter. Similar to how Vala is tied to GObject/GTK. Swift sits here, as does Xojo, of course. C# is arguably on the threshold, though I’d still classify it as a platform language. Comparing these to “major” languages isn’t particularly useful; where they sit in some ranking is strongly tied to their platform’s performance. For an enterprise, these considerations may be important as they determine supportability and availability of developers, but for smaller/individual developers, it’s more about what tool gets the job done.