[web-animations-1] Add `getAnimations()` to `ShadowRoot`

[birtles]

Currently the following will return an empty array:

const shadow = host.attachShadow({ mode: 'open' });
const elem = document.createElement('div');
shadow.appendChild(elem);
elem.animate(...);

// Get the animations from the host
// --> Returns empty array because getAnimations does not cross shadow-tree boundaries
host.getAnimations({ subtree: true });

To make working with shadow trees easier we could add:

partial interface ShadowRoot {
  sequence<Animation> getAnimations();
};

(Or should we add this to DocumentFragment instead?)

[emilio]

Is it worth it to have it in DocumentFragment? No fragment other than shadowroot can ever be connected to the document...

But if so, maybe worth having it in ContainerNode? That'd handle all of {Document, DocumentFragment, Element}.

[birtles]

But if so, maybe worth having it in ContainerNode? That'd handle all of {Document, DocumentFragment, Element}.

Yeah, we want different behavior for Document and Element however (Document doesn't need the { subtree: true } flag since it should always be true there). I'll just add it to ShadowRoot for now.

[emilio]

Maybe worth just using the DocumentOrShadowRoot mixin then?

[birtles]

Maybe worth just using the DocumentOrShadowRoot mixin then?

That sounds perfect. Thanks!