The Flex Builder properties palette only allows a “default application” class to be set in the default (a.k.a untyped) package. WTF.
So, you have to manually edit the .actionScriptProperties file to get the friggin built in compiler to work.
This was not fixed in FB 4 Beta.
I do not appreciate Adobe making architecture decisions for me. Boooo.
.actionScriptProperties with manual update:
<?xml version="1.0" encoding="UTF-8"?>
<actionScriptProperties mainApplicationPath="com/foo/bar/Main.as" version="3">