When the jkernel/deployment changes from 6u10 were forward-ported to jdk7 then System.initializeSystemClasses was hacked to workaround issues encountered with the jkernel download manager executing concurrently with the initialization. See:

http://hg.openjdk.java.net/jdk7/jdk7/jdk/diff/9053bcc8eef0/src/share/classes/java/lang/System.java

The changes for 6612680 mean that the download manager won't start until the system classes have been initialized and so the changes to System.initializeSystemClasses can be restored.