Annotations are not present in classes compiled by the Groovy compiler running on App Engine. This is because the Java version check in VMPluginFactory doesn't work on App Engine, because ClassLoader.getSystemClassLoader() returns null there. Therefore I propose to change ClassLoader.getSystemClassLoader().loadClass(JDK5_CLASSNAME_CHECK) to VMPluginFactory.class.getClassLoader().loadClass(JDK5_CLASSNAME_CHECK). The new check is equally safe and also works on App Engine. If there are no objections, I will commit this change.
Speaking of version checks, it might be a good idea to introduce (or even copy) something like Spring's JdkVersion class into the Groovy codebase. The Spring folks know very well how to handle such stuff.