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!

This is a semantic change isn't it? The old code caught the case where proto.__proto__[name] was set but proto[name] did not exist. The new code does not.