Use a task source when dispatching EventSource events. #4506

This change specifies that the DOM manipulation task source should be used
when dispatching events interpreted an EventSource stream.  This is an
incremental change towards specifying the source for all queue-a-task
instances.

Author: wanderview

source

@@ -128272,9 +128272,9 @@ any-char      = %x0000-0009 / %x000B-000C / %x000E-10FFFF
    <li><p>Set the <var>data</var> buffer and the <var>event type</var> buffer to
    the empty string.</p></li>
 
-   <li><p><span>Queue a task</span> which, if the <code
-   data-x="dom-EventSource-readyState">readyState</code> attribute is set to a value other than <code
-   data-x="dom-EventSource-CLOSED">CLOSED</code>, <span
+   <li><p><span>Queue a task</span> on the <span>DOM manipulation task source</span> which, if the
+   <code data-x="dom-EventSource-readyState">readyState</code> attribute is set to a value other
+   than <code data-x="dom-EventSource-CLOSED">CLOSED</code>, <span
    data-x="concept-event-dispatch">dispatches</span> the newly created event at the
    <code>EventSource</code> object.</p></li> <!-- so calling close() drops the messages immediately.
    The connection dying doesn't interfere with already-sent messages since it sets us to CLOSED via