Unfortunately I am very unhappy with the Null-Safety implementation of generated DTO files from ServiceStack. All properties are declared nullable which means that one has to deal with tons of if()s and clutter the entire code with ! almost everywhere.
Dart sees nullable types as an exception (see this document). I can imagine, that it is not so easy to write a code generator, especially since the topic is new and lots of people struggle and fight with it (like I do still very often).
Maybe a solution could be this package provided by Google. In the sample you can see, that it is able to deal with non-nullable class members.
It uses the build_runner package to generate the code.
I don’t know what you guys think, but for me it is currently easier to use the http package to fire my REST calls and do serialization into my client models myself.