flow: Ignore a Flow "error" that is not one

Flow warns us that 'EventTarget is incompatible with Node'. But
that is wrong. In fact the definition in Flow's code is this:

```js
declare class Node extends EventTarget {
```

Ignore!

src/webview/js/js.js

@@ -406,6 +406,7 @@ const handleLongPress = (e: TouchEvent) => {
   sendMessage({
     type: 'longPress',
     target: e.target.matches('.header') ? 'header' : 'message',
+    // $FlowFixMe EventTarget is incompatible with Node (no it is not!)
     messageId: getMessageIdFromNode(e.target),
   });
 };