cd workspace (where workspace is your Eclipse Neon Workspace)
git clone https://github.com/BobHanson/java2script.git
git checkout swingjs

Then, open Eclipse, open Navigator, and hit Refresh. java2script will show up as a blank generic trunk without a name (i.e. 'trunk[]')

Or, do the equivalent tasks on Eclipse's graphical git tool.

Go to net.sf.j2s.core directory, Clean Project, then export as a Deployable plug-ins and fragments.

Core is the only thing that was affected. Hence, everything else can be built the same exact way it was built before (assuming you were using the HEAD of the master branch of j2s).