Allows the page-attachment and share-menus to be imported inline in the document on example stories so that the extensions load on Firebase.

There's a bug where extensionsFor.installExtensionForDoc doesn't point to the right URL for the extension on Firebase demos (it points to a URL with rtv, but it should point to the dist/ url).

Relies on upgradeCallback, which on the documentation says: "This method is called when the element is added to DOM for the first time and before buildCallback to give the element a chance to redirect its implementation to another BaseElement implementation. The returned value can be either null or undefined to indicate that no redirection will take place; BaseElement instance to upgrade immediately; or a promise to upgrade with the resolved BaseElement instance."

Alternatives

An alternative can be to directly change the upgradeCallback of page-attachment and share-menu to return a promise:

// amp-story-page-attachment.js

upgradeCallback() {
    const storyEl = closestAncestorBySelector("amp-story");
    return whenUpgradedToCustomElement(storyEl)
        .then(() => storyEl.getImpl())
        .then(() => null);
}

However, this requires all the different classes to have more repeated code.

Another alternative is to do #37331 but it's harder to follow the promise chains.