blacklist problematic DOM property names

fixes https://github.com/Polymer/platform/issues/88

Thoughts @sjmiles @sorvell ?

The docs also list `focus` and `children`. I know people have had problem with `children` before.

 `focus` is actually a method, and from this text and the referenced bug, it appears that maybe only properties are an issue? In which case, the text in the docs should be updated.