Actionscript 3 :: Add Setter In A Prototype In Flex

In Flex, you can add functions to the prototype of a Class; but how do you add a setter? For example, with A some (non-dynamic) class, you can do this:

var o:Object = new A();
A.prototype.myFunction = function():void{trace("foo");}
o.foo();

And that will call the foo function. But how could you add a setter, so that setting the property calls the setter (just like it would if you declared the setter in the "normal" way on the A class). So what I want is something like this:

// doesn't work!
A.prototype["set myProperty"] = mySetter;
o.myProperty = "test"; // should call mySetter

PS: Manipulating the prototype is an unusual thing to do in Flex, and not something I'd recommend in general. But for the sake of this question, just assume that there is a reason to dynamically add a setter.