Fix a bug in `Node#insertBefore` where `connectedCallback` was being called on certain elements that were already connected.
Fix a bug in `Node#insertBefore` where `connectedCallback` was being called on certain elements that were already connected.

Previously, *all* custom element descendants of the context node would have
their `connectedCallback` called. This commit corrects this behavior such that
only the newly inserted custom elements and their descendants have their
`connectedCallback` called.

webcomponents/polyfills,webcomponents/polyfills,webcomponents/polyfills