Can I use flutter dart with ODK?

I want to download forms into dart mobile app, edit, delete and update the forms. Is this possible? Dart will run on both android and IOS.

Short answer - hypothetically, yes! Long answer - no, at least probably not in the way that you're thinking.

ODK is a collection of applications designed to enable mobile data collection. Most people engage with these applications through forms that they design using tools provided by ODK. As a result, this can create the perception that these forms are independent pieces of code or applications in and of themselves. They're not though - they're just a way of telling ODK applications what to display, or how to save data. An ODK form only truly exists within the ODK ecosystem (or other apps compatible with ODK, like KoboCollect). For instance, ODK Collect is an android application that displays ODK forms and allows them to be filled.

Dart is a programming solution designed to allow developers to write cross-platform applications. There's nothing about Dart that really has anything to do with ODK forms themselves - again, those forms aren't applications or really code in and of themselves. Now, if you were to port the entire ODK Collect application into Dart, so as to create a cross-platform version of ODK Collect, then yes, you would have created a way to fill ODK forms both on iOS and Android. However, this would be done not by doing anything with the forms themselves, but by creating a whole new application compatible with these forms that can run on iOS.

This is something you can do, as ODK is open source! However, this would be a huge undertaking. If you really need forms to be fillable on iOS, I believe there are already iOS apps out there that are compatible with ODK forms (a quick google gave me, but I have no experience with it and so won't vouch for it).