I have created the tree view of Flex class hierarchy in flash.The root class is Object and other classes extend it. You have to click on the folder icon to see its subclasses.

Flex Class reference

You can see it in action at the following link.

http://www.ashwineedash.com/template_permalink.asp?id=114

Also you can get the download link from there.

It is specifically helpful if you want to know the class structure without going for help files.