Missing do construct

No benefit to excluding it, and trying to use it throws a parse error because of the prepended return.

Author: Roy-Orbison

src/WP_CLI/Shell/REPL.php

@@ -63,6 +63,7 @@ private static function non_expressions() {
 				'global',
 				'unset',
 				'function',
+				'do',
 				'while',
 				'for',
 				'foreach',