Flex Formatter is one of my favorite plugins for FlashBuilder, unfortunately there's no plugin version for IntelliJ.
But thanks to IntelliJ's "External Tools", you're able to use any tool that can be run from the command line (think apparat, sprouts, etc) and execute it with a keyboard shortcut within IntelliJ! The video below shows you how:
Running FlexFormatter from the Command Line:
Download the files below:
Sample Formatter.properties file: