Clearing cache does not immediately reload theme's .info file

To duplicate:

1) Open the current theme's .info file
2) Modify the stylesheets[all][] line and change the style.css file read a non-existent file (nostyle.css)
3) Empty the cache on the Performance page
4) Notice the theme does not change
5) Click around through various pages on the site. Some pages will finally cause the new .info file to load (apparently triggered by something in the menu system forcing a cache reload) and the site will go themeless.

This patch places a call system_theme_data in the drupal_flush_all_caches() function to make force the data in the system table which holds the .info file to reload immediately.