Bootstrap: Revert [38399] as it's broken /build/ and subsequently core.svn.wordpress.org.

The generated classmaps reference `/src/` files and operates in the assumption that the base directory is one level above `wp-settings.php`, which it isn't after our build processes are run.

See #36335


git-svn-id: https://develop.svn.wordpress.org/trunk@38402 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/customize/class-wp-customize-selective-refresh.php

@@ -67,6 +67,7 @@ final class WP_Customize_Selective_Refresh {
 	 */
 	public function __construct( WP_Customize_Manager $manager ) {
 		$this->manager = $manager;
+		require_once( ABSPATH . WPINC . '/customize/class-wp-customize-partial.php' );
 
 		add_action( 'customize_preview_init', array( $this, 'init_preview' ) );
 	}