Error with polymer element

Fluent http can't serve html pages contains polymer element.

An error message appears in console

```
inline@1ba5821b:52:57: could not find helper: 'checked'
      <div id="checkbox" class$="[[_computeCheckboxClass(checked)]]">
```

A simple project with this issue is present on this repository
https://github.com/olive2bx/raspberry-pi-gpio-web-gui
