Data classes, like in Kotlin, are very useful. It's a very common pattern in modern systems and very well-known (it's redundant for modern developers to explain it here).

In Java, people uses https://projectlombok.org/ to have it. In many other languages is programmed in a normal project code to mimic the behaviour, but the best way is to have it as a language feature.

Dart already have incorporated interesting and important features to the language, increasing productivity, but is still missing this one.

Also there's a popular feature request at GitHub, that exists for about 3 years that was ignored:
https://github.com/dart-lang/language/issues/314

Dart and Flutter are getting even more popular theses days and is time to have this important and productive feature in the language!

Let's vote for it!