Declare a class inside a function in dart

I'm building a library for an API that has hundreds of API functions and result types. For this reason, I made a simple generation script that should create all these types from a definition file.

This is how it was in javascript :

const params = parseTypes();
const api = {}
for (const classParams of params) {
    class VirtualClass{
        className = classParams.name;
        classType = classParams.type;
    }
    api[classParams.name] = VirtualClass;
}

Which worked fine for its purpose but when I tried to implement the same strategy in Dart I couldn't declare a class inside a function. doing the following

void main() {
  class A{
  }
}

gives a lot of red lines and errors

Error: Expected an identifier, but got 'class'.
  class A{
  ^^^^^

Is there a way to implement the same idea in Dart?