Upgrade: New themes are not automatically installed on upgrade. This can still be explicitly asked for by defining CORE_UPGRADE_SKIP_NEW_BUNDLED as false.

In `populate_options()`, if the theme specified by `WP_DEFAULT_THEME` doesn't exist, fall back to the latest core default theme. If we can't find a core default theme, `WP_DEFAULT_THEME` is the best we can do. 

Props nacin, jeremyfelt, dd32.
See #34306.


git-svn-id: https://develop.svn.wordpress.org/trunk@35738 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/update-core.php

@@ -715,6 +715,9 @@ $_old_files = array(
  * Directories should be noted by suffixing it with a trailing slash (/)
  *
  * @since 3.2.0
+ * @since 4.4.0 New themes are not automatically installed on upgrade.
+ *              This can still be explicitly asked for by defining
+ *              CORE_UPGRADE_SKIP_NEW_BUNDLED as false.
  * @global array $_new_bundled_files
  * @var array
  * @name $_new_bundled_files
@@ -732,6 +735,11 @@ $_new_bundled_files = array(
 	'themes/twentysixteen/'  => '4.4',
 );
 
+// If not explicitly defined as false, don't install new default themes.
+if ( ! defined( 'CORE_UPGRADE_SKIP_NEW_BUNDLED' ) || CORE_UPGRADE_SKIP_NEW_BUNDLED ) {
+	$_new_bundled_files = array( 'plugins/akismet/' => '2.0' );
+}
+
 /**
  * Upgrade the core of WordPress.
  *