AS3 :: Flash - Get All Subclasses Of A Class?

This is a little odd but I need to know all subclasses of a specific class. So let's say I have the base class called Shape and 3 subclasses (Circle, Triangle and Rectangle) that implement it. I would like something like this :

public function getSubclasses(aClass : Class) : Vector.<Class>
{
//???
}

that returns a vector containing Circle, Triangle and Rectangle.I unfortunately cannot count on the fact that each subclass be instanciated at least once before that function is called.t the moment I need to manually add the classes to a list but this is error prone (if I forget) and also not as versatile as the function proposed above would be (since I would need to make a new list for every class I want the subclasses of).