Source

File: wp-admin/includes/class-wp-upgrader.php

protected function flatten_dirlist( $nested_files, $path = '' ) {
		$files = array();

		foreach ( $nested_files as $name => $details ) {
			$files[ $path . $name ] = $details;

			// Append children recursively.
			if ( ! empty( $details['files'] ) ) {
				$children = $this->flatten_dirlist( $details['files'], $path . $name . '/' );

				// Merge keeping possible numeric keys, which array_merge() will reindex from 0..n.
				$files = $files + $children;
			}
		}

		return $files;
	}