Flash Builder not auto completing

I imported an existing project into FlashBuilder 4.0 and found that content assist (and class linking) didn't work.  After a bit of googling and testing it turns out that when you import a project the compiler looks for the main classes in the src folder. If you have them in a different place you need to explictly set this by right clicking the project > properties > Actionscript Build Path and then set the "Main Source folder" to the custom location (it will be set to src by default).