Fix for document.createElement('paper-button') DOMException

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[jaichandra]

I signed the CLA.

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[jaichandra]

@lychyi My code is now broken due to the changes in 2.0 branch. Will refactor and check. but yeah.. as per docs, for legacy elements it should be attached. Demo works with attached.

[jaichandra]

attached works.

[lychyi]

Fantastic @jaichandra nice work! Everything looks good on my end. Hopefully the team can get this merged soon. I ran into this when extending paper-button with the new 2.0 class syntax and it caused quite a headache.

[notwaldorf]

This is no longer needed, since we're not putting the paper-material class on the host anymore