adoptNode was only stubbed on the createHTMLDocument shim object, never on
Document.prototype, so document.adoptNode(node) threw. importNode already had a
Document.prototype polyfill returning the node, so adoptNode mirrors it: with no
second document to transfer ownership from the node is already ours, return it.
toggleAttribute was missing entirely. Implement spec semantics: no force arg
toggles, force true adds, force false removes, returns the new presence. Lit,
Stencil, and several ad SDKs call it and die on the missing method today.
Fixes#393.