Index: src/wp-includes/plugin.php
===================================================================
--- src/wp-includes/plugin.php	(revision 29434)
+++ src/wp-includes/plugin.php	(working copy)
@@ -653,6 +653,8 @@
 function plugin_basename( $file ) {
 	global $wp_plugin_paths;
 
+	$file = wp_normalize_path( $file );
+
 	foreach ( $wp_plugin_paths as $dir => $realdir ) {
 		if ( strpos( $file, $realdir ) === 0 ) {
 			$file = $dir . substr( $file, strlen( $realdir ) );
@@ -659,7 +661,6 @@
 		}
 	}
 
-	$file = wp_normalize_path( $file );
 	$plugin_dir = wp_normalize_path( WP_PLUGIN_DIR );
 	$mu_plugin_dir = wp_normalize_path( WPMU_PLUGIN_DIR );