add $$host

[tanhauhau]

Fixes #3091

• compile error if using $$host without using customElement: true

[wyozi]

Is there a strong reason to restrict $$host to just custom elements? I admit the usecases for it outside custom elements may be few, but I have one: a wrapper component for testing components that rely on context. The test wrapper component will wrap a provided with a context, while also passing rest of the props/callbacks from the host to the wrapped component using the $$host reference.

Accessing the host component is doable with get_current_component, but a stable non-internal API wouldn't hurt.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[tronicboy1]

This change is really cool! If I am correct it will give use access to this of the custom element eh?

Very exciting and thank you for this!

[Rich-Harris]

Closing as it appears we ultimately decided against this. I've just opened #11022 though