Fix for core-dropdown "Cannot read property 'position' of null"

[dylanPowers]

I was impacted by the bug reported at https://github.com/Polymer/core-dropdown/issues/11. Someone graciously created a quick fix at https://github.com/Polymer/core-dropdown/pull/14, but it has yet to be merged (don't know why). After some quick testing it works great. I don't know if you'd rather that be merged first and ignore this request, but either way it needs to be merged in somewhere as this was giving my app troubles.

[jakemac53]

We usually wait until they at least get merged upstream, otherwise we can be left in a weird state if they decide not to merge it for some reason. As soon as that is merged we can merge it here though, if you ping this again once its ready we can merge it.