Currently, there is no way to create an ActionScript-based AIR project. There is a workaround where you create a Flex-based AIR project and remove the XML file and add a main class file instead and set that as your main application, or change the name of the file from .mxml to .as and make it a class. But to someone who doesn’t know this, it would appear that AIR == Flex only.
If you think it’s important to have this option, vote: