This online tool is a great tool for programming with Dart and Flutter.
The author named Paul Mutisya tell us:
I’ve updated the Flutter Studio web application with the goal of making it more flexible, complete, responsive and accurate. The app should now work better on different screens, keep you in the flow, accurately display your app on multiple devices and orientations and work consistently. In addition, it now produces complete, working code and build files and has a more complete set of widgets, including some new ones not found in the flutter.io widget catalog.
The new version
Shows real Android and iOS devices accurately on the web
Produces and shows pixel perfect designs
Provides more widgets (including themes)
Allows you to edit the widgets intuitively, producing accurate code
Focuses on design (code and pubspec are on separate tabs)
Produces complete, working application code and pubspec.yaml files