I've looked into Dart a few times and thought it looked interesting, but never really dug much into it. I've also really only heard of it being used in Google. Oh, and I remember hearing Notch was using it in a game jam a few years back.
I saw Flutter when it was first announced, and something that caught my eye back then was that it looked like it took some inspiration from React. I'm a big fan of React. Their home page [0] says: "Note: Flutter is an early-stage open-source project. We hope you try Flutter out and send us feedback." Would you say it's ready for production?
Could you provide other examples of use-cases for which you'd use Dart? Does it have good editor integration for autocompletion and linting? Is debugging painful? One of my favorite things about JS is Chrome Dev Tools.
I don't mean for this to come off as rude or negative, but I'd be interested in hearing a bit about where you consider the language or ecosystem falls short. I find discussing the downsides, limitations, and problems helps in understanding the tool and setting expectations. Nobody expects a magic wand solution, of course.
One of my pain points from working with a large SPA is architecting the application so it's easy to lazily views as needed. By itself it's not very difficult, but getting the whole build / development / test environment workflows setup is more challenging to get right. Does Dart have good tooling for generating builds? I use Webpack with the large JS SPA I work on, and one of its killer feature is that it'll crawl the dependency graph, copying assets to the output folder, and updating references. Unfortunately, the default behavior makes it most easy to generate a single large JS bundle.
[0] https://flutter.io/