Instantiate a class from a string in ActionScript 3

I've got a string which, in run-time, contains the name of a class that I want to instantiate. How would I do that?

I read suggestions to use flash.utils.getDefinitionByName():

var myClass:Class = getDefinitionByName("package.className") as Class;
var myInstance:* = new myClass();

However, that gives me the following error:

[Fault] exception, information=ReferenceError: Error #1065: Variable className is not defined.

The easiest method I've come up with is to simply write the classnames out, separated by semicolons, anywhere in your project.

e.g. I create an Assets.as file with this in it:

package {   

public class Assets {       

    // To avoid errors from the compiler when calling getDefinitionByName
    // just list all of the classes that are not otherwise referenced in code:
    Balloon;
    Cloud;
    FlyingHorse;
    FlyingPig;
    UFO;
    Zeppelin;       
}
}

Full code example/tutorial on this is here: http://producerism.com/blog/flashpunk-dame-and-lua-tutorial-part-6/