This page describes how to get the latest source from Subversion. Note, that the version in Subversion may not be as stable as the current release build.
You can download a zip file that is created daily that contains the latest source from here.
svn at the commandline. If you don’t have it, then download and install it. svn checkout http://svn1.cvsdude.com/osflash/flashjavascript/
This will create a directory called “flashjavascript” that contains the latest source.