BroadcastChannel: move destination close flag

This way nothing happens if something got closed during delivery. (Implementations already did this.)

Tests: web-platform-tests/wpt#21895.

Fixes #1371. Possible follow-up: #5327.

source

@@ -100430,9 +100430,6 @@ interface <dfn>BroadcastChannel</dfn> : <span>EventTarget</span> {
 
      <li><p>Their <span>channel name</span> is a <span>case-sensitive</span> match for
      <var>sourceChannel</var>.</p></li>
-
-     <li><p>Their <span data-x="concept-BroadcastChannel-closed">closed flag</span> is
-     false.</p></li>
     </ul>
    </li>
 
@@ -100454,6 +100451,9 @@ interface <dfn>BroadcastChannel</dfn> : <span>EventTarget</span> {
     object</span>'s <span>responsible document</span>.</p>
 
     <ol>
+     <li><p>If <var>destination</var>'s <span data-x="concept-BroadcastChannel-closed">closed
+     flag</span> is true, then return.</p></li>
+
      <li><p>Let <var>targetRealm</var> be <var>destination</var>'s <span
      data-x="concept-relevant-realm">relevant Realm</span>.</p></li>