Announcing Dart 2.3: Optimized for building user interfaces
medium.com
medium.com
I am not sure why dartlang being an open source language support proprietary editors like vscode, indirectly forcing them to be snooped by Microsoft and others. Hopefully the team will give equal support to open source editors Emacs and Vim for dart language in the name of security and privacy as they were promoting it in google IO 2019.
Short answer: there are a lot of different editors in the world, but Dart team is not large enough and does not posses necessary expertise to develop top-notch integration with all of them. So it makes sense to focus on the very popular ones (and both VSCode and IDEA are very popular) and provide very good support for those.
We hope that community can take and support other editors. That is why we provide tools (e.g. dart analyzer which has an LSP wrapper) that make it easy to build Dart plugins for any editor you like to use.
There are plugins for both emacs (https://github.com/bradyt/dart-mode) and vim (https://github.com/dart-lang/dart-vim-plugin), which I think work relatively well, but they are not maintained by the Dart team.