I have successfully created (imported) a Flex 3 project into Flash Builder 4 Beta 2. I have enabled the "Use Flex 3 compatibility mode" checkbox and set the Component Set to "MX Only" as I do not use any Spark Components. The project works fine. (lets call this the Parent app)
I have built another Spark based swf using Flash Builder 4 Beta 2 as well. This has the Component Set as "MX + Spark". (Because it uses the new Spark Components). I have not turned on "Use Flex compatibility mode" (checking this actually causes errors on exporting a release build). So, I successfully export this new Spark based swf file. (lets call this the Child app)
Now, when I try to load the child app (using the Spark components) into the Parent app I get "compatibility errors". Strange that the Parent app won't even let me load (Loader class) the Child app inside it. These errors also occur if I change the Parent app to include the "MX + Spark" Component Set. (although the app doesn't use the Spark set)
Here are the error messages when attempting to incorporate the Child app into the Parent app:
Error: Compatibility version has already been set.
at mx.core::FlexVersion$/set compatibilityVersion()[E:\dev\gumbo_beta2\frameworks\projects\framework\src\mx\core\FlexV ersion.as:242]
at mx.core::FlexVersion$/set compatibilityVersionString()[E:\dev\gumbo_beta2\frameworks\projects\framework\src\mx\core \FlexVersion.as:301]