Re: [w3c/webcomponents] Custom Element - untrackable upgrade (#671)

>  it would reject the promise instead of resolving it later when it gets upgraded.

just to be sure I understand ... considering the following code:

```js
customElements.define('be-fore', class extends HTMLElement {});

const tp = document.createElement('template');
tp.innerHTML = '<be-fore></be-fore><af-ter></af-ter>';

// are you saying this will resolve ...
customElements.whenDefined(tp.content.firstChild).then(console.log);

// ... but this will throw rught away ?
customElements.whenDefined(tp.content.lastChild).catch(console.error);

// since the element is defined after ?
customElements.define('af-ter', class extends HTMLElement {});
```

This is a caveat with CE that might work for me, but it's hard to explain together with the current `whenDefined` mechanism, where you ask for a CE not defined yet.

To solve this though, the following might work as well:

```js
function setDirectValue(node, value) {
  customElements
    .whenDefined(node.nodeName.toLowerCase())
    .then(() => {
      customElements.whenDefined(node)
        .then(() => {
          node.direct = value;
        });
    });
}
```

This works for me.



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