Fix typo in heartbeat.js. see #23216.

git-svn-id: http://core.svn.wordpress.org/trunk@24139 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Compress scripts/styles: 3.6-beta1-24140.

git-svn-id: http://core.svn.wordpress.org/trunk@24140 1a063a9b-81f0-0310-95a4-ce76da25c4cd