First of all: how great that JetBrains is making a competitor for Flash Builder. Given how terrible Flash Builder is, it will come as a great relief to have the people behind IntelliJ release an alternative.
I have a question.
I collaborate and version by putting my source files on Dropbox and all binaries on a local folder. This is all working the same as in IntelliJ (add a content root). However I do not know how to tell Astella to use the main file (say Main.mxml) from this content root.
there's 'Main class' field in Astella build configuration (under module node in Project Structure dialog). So just write 'Main' there and make sure that Main.mxml file is under source folder. Content roots and source folders are configured at module level. Next EAP will have main class completion and validation in respective field.