Also I think having a highly integrated toolchain is really important for this. Building something cross-platform with Flutter feels just like a fresh breeze even for web which is a beta target (Desktop being alpha). I would have never gotten the idea to write a web app in Dart, but with Flutter it feels just fine. Like Typescript but from Google.
Speaking of the web, angulardart got a major update a few weeks ago. And on the subject of web-app-on-dart, my personal site is written in dart (and the 'platform', as some people would call it, is open source for that matter if someone wants to use it or contribute).
