polyfilled "invalid input" error bubbles no longer work in Safari with Polymer paper-inputs

[jab]

Hi @aFarkas,

I wanted to let you know about an issue I discovered using webshim's form polyfill in Safari 9.1 with Polymer paper-inputs. Details are in PolymerElements/paper-input#375.

On the off chance you have a clue about what's causing this, any tips would be greatly appreciated.

Thanks, and thanks for all your work on webshim!