Update Nov 2018:
It would appear that Structure101 (new download page) no longer has an ActionScript variant.
Original answer, links outdated:
Download Structure101g and select the Actionscript flavor after installing the software.
I’ve confirmed that it is able to map out class level and even function call dependencies in Flex/AS3 projects, and generate a visual map of the same.
Take a look at the attached screenshot.
(non-SSL link dead: i40.tinypic.com/e8qptu.png)
Hope that helps.