How to get a property by this name in String?

Unfortunately, you cannot use reflection/mirrors in flutter. What you can do, which is tedious, is use maps.

class PrefsState { 
   String a; 
   const PrefsState({ this.a, });
   dynamic getProp(String key) => <String, dynamic>{
    'a' : a,
    }[key];
}

It's probably better to build the map in the constructor, but if you want const constructors then you'll have to settle for this. Likely won't make much of a difference unless you have a million parameters anyway. Then you use it like so:

PrefsState test= PrefsState(a: "it is a test");
String key = "a"; 
print(test.getProp(key));

I don't think there is a less cumbersome way of doing this, but would love to be proven wrong :-)