Contained elements are never fully adopted and always match :unresolved

[depeele]

Commit 8697b40 which was used to close issue #231 causes issues with contained custom elements.

Contained custom elements are never fully adopted even when the containing element is finally adopted. This means that contained elements will always be matched by the :unresolved CSS pseudo-property.

In https://codepen.io/depeele/pen/QBdZyo using lit-html:v0.10.0 the contained test-item is considered resolved in the final document.

In https://codepen.io/depeele/pen/PBWyzB using lit-html:v0.10.1 but otherwise the same code, test-item is not considered resolved in the final document.

[depeele]

Turns out, the WebComponents standards committee decided to rename :unresolved to :not(:defined) (w3c/webcomponents issue 418).

See https://codepen.io/depeele/pen/gjGPRy which uses lit-html:v0.10.1 and the new pseudo-property.