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!

thinking on this more: the closest thing to the original behavior would be to get the descriptor (from any prototype) and see if it has a getter or value. The problem is what to do in a case where we had a getter? The original code was calling it, but with the wrong `this`, so for DOM accessors they won't work. Also a getter written in code might not work either.

We could only take the value if the descriptor has a .value, but I seem to recall that broke some tests as well.