[polymer][preview-0.8] No more custom element extends?

[Cayter Goh]

I've just gone through the PRIMER.md here. It's mentioned there that the 0.8 currently only supports extending native HTML elements. I'm wondering whether the statement means that this will stay the same when the polymer team releases the official 0.8. If is is so, how does the team deal with the paper-elements that has dependencies on core-elements? By rewriting them? Thanks.