Updated my localhost Moodle 26(Alpha) branch last night. Went to do upgrade this morning and found this error message.

! ) Fatal error: Class 'core_useragent' not found in C:\wamp\www\moodle26\lib\pagelib.php on line 1552
Call Stack

1. Time Memory Function Location
   1 0.0027 840096 {main}

   ( ) ..\index.php:0
   2 0.3257 33801952 moodle_page->get_renderer( ) ..\index.php:100
   3 0.3257 33802000 moodle_page->magic_get_theme( ) ..\pagelib.php:772
   4 0.3257 33802000 moodle_page->initialise_theme_and_output( ) ..\pagelib.php:635
   5 0.3257 33802048 moodle_page->resolve_theme( ) ..\pagelib.php:1466