Why are there two ArrayList classes?

[Gareth Smith]

Hi,

I am putting together a patch for ArrayList to support square-bracket 
indexing and for-each loops, see: 
https://issues.apache.org/jira/browse/FLEX-15159

I am confused by the existence of not one, but two ArrayList classes: 
the "original" one at mx.collections.ArrayList and another one at 
org.apache.flex.collections.ArrayList.

Why are there two classes? Looking at their diff, I would guess that 
mx.collections.ArrayList is newer: it has some additional optimisations, 
and an additional toJSON() method.

I could patch both classes, but I would rather not introduce more 
duplication that might become a maintenance burden along the line.

Thanks
Gareth