Object.getNotifier() part of Polymer webcomponents polyfill?

Question

I have a Polymer app that runs in Chrome, but in Firefox and Safari I get the error TypeError: Object.getNotifier is not a function

I included the webcomponents.min.js polyfill library (which includes observe-js), which I thought polyfilled this functionality. Is there something I'm missing?

We're using Polymer v0.5.5

Thanks!