[dom] Pull Request: `document.importNode` and `document.adoptNode` should throw exceptions when acting upon ShadowRoots

samgiles has just submitted a new pull request for 
https://github.com/whatwg/dom:

== `document.importNode` and `document.adoptNode` should throw 
exceptions when acting upon ShadowRoots ==
The `importNode` algorithm relies on a `clone` of `node` however, the 
Shadow
DOM spec stipulates that cloning a `ShadowRoot` should raise a
`DataCloneError`. Consequently, importing a `ShadowRoot` should
be unsupported.

Additionally, it doesn't make sense for a document to `adopt` a 
`ShadowRoot` -
the shadow host and `ShadowRoot` should not have different documents.
  In this
instance a `HierarchyRequestError` seems to make sense.

See https://github.com/w3c/webcomponents/issues/125

Using `bikeshed`:

Note:  I ran `bikeshed update && bikeshed spec` however the updated 
`dom.html` had spurious other updates to anchors unrelated to this 
change - first time using `bikeshed`, have I done something wrong 
here?

See https://github.com/whatwg/dom/pull/46