Re: [whatwg/dom] MutationObserver needs an equivalent of disconnected & connected callbacks (#533)

> This would be useful for polyfilling custom elements

polyfills for custom elements already work and are either based on MutationObserver or DOMNodeInserted/DOMNodeRemoved with a lookup for registered elements in the list of changed nodes.

Using polyfills is a weak reason, specially because anythig new needs to be polyfilled too.

**However** I do have a use case in hyperHTML too but I believe React and many other frameworks would love this feature as well.

CustomElements mechanism to understand their life-cycle are excellent but not everyone writes Custom Elements based sites and knowing a generic form, input, list element, grid card, has been added/removed would be ace.

in hyperHTML I do have control of each vanilla DOM component but I need to do this "dance" per each event:
```js
function dispatchTarget(node, isConnected, type, e) {
    if (components.has(node)) {
      node.dispatchEvent(e || (e = new Event(type)));
    }
    // this is where it gets a bit ugly
    else {
      for (var
        nodes = node.children,
        i = 0, length = nodes.length;
        i < length; i++
      ) {
        e = dispatchTarget(nodes[i], isConnected, type, e);
      }
    }
    return e;
  }
```

The dispatching after recursive search is done per each `record.addedNodes` and `record.removedNodes` but if I had a platform way to loop over all known connected/disconnected nodes my code, and my life, would definitively be easier.



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