It's in jboss-ejb3-embedded-sub.jar which should be located in $JBOSS_HOME/common/lib (starting with rev 109912).
JBossStandaloneEJBContainerProvider will create a class loader that includes $JBOSS_HOME/lib, $JBOSS_HOME/common/lib, $JBOSS_HOME/server/${embedded.server.name}/lib and $JBOSS_HOME/client (+ a JCA hack). From this class loader JBossSubmersibleEJBContainer is loaded, which in turn loads JBoss Embedded AS.