Add attribute changed steps to dialog element

When the dialogs open attribute is removed:

1. Remove dialog from the document's open dialogs list.
2. Destroy and nullify dialog's close watcher

This also adds an assertion to the start of
'set the dialog close watcher' that dialog's close watcher is null.

# Conflicts:
#	source

source

@@ -62110,6 +62110,9 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
   element <var>dialog</var>:</p>
 
   <ol>
+   <li><p><span>Assert</span>: <span>dialog</span>'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
     result of <span data-x="establish a close watcher">establishing a close watcher</span> given
@@ -62222,6 +62225,38 @@ interface <dfn interface>HTMLDialogElement</dfn> : <span>HTMLElement</span> {
    <var>removedNode</var>'s <span>node document</span>'s <span>open dialogs list</span>.</p></li>
   </ol>
 
+  <p>The following <span data-x="concept-element-attributes-change-ext">attribute change
+  steps</span>, given <var>element</var>, <var>localName</var>, <var>oldValue</var>,
+  <var>value</var>, and <var>namespace</var> are used for <code>dialog</code> elements:</p>
+
+  <ol>
+   <li><p>If <var>namespace</var> is not null, then return.</p></li>
+
+   <li><p>If <var>localName</var> is not <code data-x="attr-dialog-open">open</code>, then
+   return.</p></li>
+
+   <li><p>If <var>value</var> is null:</p>
+
+    <ol>
+     <li><p><span data-x="list remove">Remove</span> <var>element</var> from <var>element</var>'s
+     <span>node document</span>'s <span>open dialogs list</span>.</p></li>
+
+     <li>
+      <p>If <var>element</var>'s <span data-x="dialog-close-watcher">close watcher</span> is not
+      null, then:</p>
+
+      <ol>
+       <li><p><span data-x="close-watcher-destroy">Destroy</span> <var>element</var>'s <span
+       data-x="dialog-close-watcher">close watcher</span>.</p></li>
+
+       <li><p>Set <var>element</var>'s <span data-x="dialog-close-watcher">close watcher</span> to
+       null.</p></li>
+      </ol>
+     </li>
+    </ol>
+   </li>
+  </ol>
+
   <p>When a <code>dialog</code> element <var>subject</var> is to be <dfn data-x="close the
   dialog">closed</dfn>, with null or a string <var>result</var>, run these steps:</p>