This plugin failed in the "package" phase for Grails 2.3.1 and 2.3.2.

By adding:

   includeTargets << grailsScript("_PackagePlugins")

to the _AssetCompile.groovy the problem goes away
removed un-needed blank line
Merge pull request #42 from sev9/master

Fix for "grails package" when using your plugin in a Grails 2.3.1 or 2.3.2 project