ActionScript 3.0 :: Delete A Dynamic Property Of A Dynamic Object?

I am using a regular Flash object as a simple data container, and I want to remove a particular property of that object. Then that object gets passed on to a for..in loop. But even after using delete to remove the property, the key still shows up in the loop.
 
For example:
 
var obj:Object = { a: "a", b: "b", c: "c" }
delete obj['b'];
for ( var prop:String in obj ) {

[Code].....
 
So deleting the property seems to be the same as assigning its value to null.
 
Is there any way to immediately remove the property from the object altogether so that it is no longer looped over in a for..in loop?
 
It is not sufficient to check the property for a null value since a property with a null value is not the same as not having the property in the first place, and represents a different state. I don't want to set the property to null, I want to remove the property.