Because the `WP_ADMIN` constant name can be bound in multiple files, all instances should check `! defined` first. `wp-admin/admin.php` already has this check. 

See #27881.

Built from https://develop.svn.wordpress.org/trunk@28478


git-svn-id: http://core.svn.wordpress.org/trunk@28305 1a063a9b-81f0-0310-95a4-ce76da25c4cd
`hackificator` complains if you call `include 'file.php'` without the parens, needs to be `include( 'file.php' )`

See #27881.

Built from https://develop.svn.wordpress.org/trunk@28479


git-svn-id: http://core.svn.wordpress.org/trunk@28306 1a063a9b-81f0-0310-95a4-ce76da25c4cd