У вашего броузера проблема в совместимости с HTML5
Curious to know how Polymer’s data binding system works under the hood? Today on Polycasts I’m kicking off a multi-part series to peel back the layers and reveal the mysteries of what Polymer does when your bindings update. Come check it out!
Demo Source
https://github.com/Polymer/polycasts/tree/master/ep28-data-binding-101/binding-demo
Setters in JavaScript
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/set
Polymer Slack: http://bit.ly/polymerslack
Subscribe to the Chrome Developers channel at: https://goo.gl/OUF4e2