For the vast majority of use cases, JavaScript (or TypeScript) is going to be a better solution than Dart.
Like it or not, JavaScript is the #1 player on the web and if you're trying to scale and share resources or code at all, you want your engineers to be proficient in JS. Yes, Dart is similar, but it's not JS. And it's not _just_ the language: it's the ecosystem. Linting, formatting, package managers, debugging issues, Googling problems, Stack Overflow answers, etc.
Also - Guess how much of that Dart specific knowledge was useful in my current role? None. It's gonna be hard to hire devs who want to work on Dart IMO.
My opinion: You can have Android, iOS, Web & Desktop with 95%+ code sharing between them using React Native + React Native Web -> https://github.com/devhubapp/devhub