Enable MessageEvent to supplant ServiceWorkerMessageEvent

See w3c/ServiceWorker#989 for discussion.

source

@@ -3885,6 +3885,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x="scope-match-algorithm" data-x-href="https://w3c.github.io/ServiceWorker/#scope-match-algorithm">match service worker registration</dfn></li>
      <li><dfn data-x="dfn-service-worker" data-x-href="https://w3c.github.io/ServiceWorker/#dfn-service-worker">service worker</dfn></li>
      <li><dfn data-x="dfn-service-worker-container-interface-client" data-x-href="https://w3c.github.io/ServiceWorker/#dfn-service-worker-container-interface-client">service worker client</dfn></li>
+     <li><dfn data-x-href="https://w3c.github.io/ServiceWorker/#service-worker-interface"><code>ServiceWorker</code></dfn> interface</li>
      <li><dfn data-x="service-worker-container-interface" data-x-href="https://w3c.github.io/ServiceWorker/#service-worker-container-interface"><code>ServiceWorkerContainer</code></dfn> interface</li>
     </ul>
 
@@ -92737,19 +92738,21 @@ interface <dfn>MessageEvent</dfn> : <span>Event</span> {
   readonly attribute any <span data-x="dom-MessageEvent-data">data</span>;
   readonly attribute USVString <span data-x="dom-MessageEvent-origin">origin</span>;
   readonly attribute DOMString <span data-x="dom-MessageEvent-lastEventId">lastEventId</span>;
-  readonly attribute (<span>WindowProxy</span> or <span>MessagePort</span>)? <span data-x="dom-MessageEvent-source">source</span>;
+  readonly attribute <span>MessageEventSource</span>? <span data-x="dom-MessageEvent-source">source</span>;
   readonly attribute FrozenArray&lt;<span>MessagePort</span>&gt; <span data-x="dom-MessageEvent-ports">ports</span>;
 
-  void <span data-x="dom-MessageEvent-initMessageEvent">initMessageEvent</span>(DOMString type, boolean bubbles, boolean cancelable, any data, USVString origin, DOMString lastEventId, (<span>WindowProxy</span> or <span>MessagePort</span>)? source, sequence&lt;<span>MessagePort</span>&gt; ports);
+  void <span data-x="dom-MessageEvent-initMessageEvent">initMessageEvent</span>(DOMString type, boolean bubbles, boolean cancelable, any data, USVString origin, DOMString lastEventId, <span>MessageEventSource</span>? source, sequence&lt;<span>MessagePort</span>&gt; ports);
 };
 
 dictionary <dfn>MessageEventInit</dfn> : <span>EventInit</span> {
   any data = null;
   USVString origin = "";
   DOMString lastEventId = "";
-  (<span>WindowProxy</span> or <span>MessagePort</span>)? source = null;
+  <span>MessageEventSource</span>? source = null;
   sequence&lt;<span>MessagePort</span>&gt; ports = [];
-};</pre>
+};
+
+typedef (<span>WindowProxy</span> or <span>MessagePort</span> or <span>ServiceWorker</span>) <dfn>MessageEventSource</dfn>;</pre>
 
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