Replace early return with assert

Author: lukewarlow

source

@@ -62515,8 +62515,8 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
   element <var>dialog</var>:</p>
 
   <ol>
-   <li><p>If <span>dialog</span>'s <span data-x="dialog-close-watcher">close watcher</span> is not
-   null, then return.</p></li>
+   <li><p><span>Assert</span>: <var>dialog</var>'s <span data-x="dialog-close-watcher">close
+   watcher</span> is null.</p></li>
 
    <li>
     <p>Set <var>dialog</var>'s <span data-x="dialog-close-watcher">close watcher</span> to the