Polymer properties with names the same as DOM accessors will now work correctly

This fixes https://github.com/Polymer/platform/issues/88

The old behavior happened to work on Chrome because Chrome is broken, and conversely failed on all of the correct browsers. Ironic. The new test adds links to the relevant bugs. (If only these bugs didn't take 4+ years to fix. Sigh.)

Thanks to @arv for the suggested fix!

yeah ... I was a little worried about that. Originally I had it searching up the proto chain.

Two things worry me about searching up the proto chain though:

We could end up using the DOM accessor as the backing store, and I'm not sure of a good way to detect that case. In other words it seems like we wouldn't want to be getting/setting HTMLElement.prototype.title as our "field".

There's also going to be a bit more logic needed to prevent finding an accessor we ourselves defined on a parent proto. That leads to stack overflow (and even if it didn't, would probably be really slow). 

Those two issues nudged me towards using getOwnPropertyDescriptor, which had the nice benefit of passing the tests without any changes. But I'm not sure it's the right call.