Specs: define registerElement() a little differently, so that the code

sky/examples/radio.sky

 SKY MODULE - radio button and radio button group
 <!-- accessibility handling not implemented yet, pending mojo service -->
 
 <script>
  module.exports = {};
 </script>
 
 <!-- <radio> -->
 <template id="radio-shadow">
  <style>
   :host { width: 1em; height: 1em; border: solid; background: white; }
   :host[checked] { background: black; }
  </style>
 </template>
 <script>
  module.exports.RadioElement = module.registerElement({
    tagName: 'radio',
    shadow: true,
-   prototype: (class extends Element {
+   constructor: class extends Element {
      constructor (module) {
        super(module);
        this.addEventListener('click', (event) => this.checked = true);
        this.shadowRoot.appendChild(module.document.findId('radio-shadow').content.cloneNode(true));
      }
      get checked () {
        return this.hasAttribute('checked');
      }
      set checked (value) {
        if (value)
          this.setAttribute('checked', '');
        else
          this.removeAttribute('checked');
      }
      get value () {
        return this.getAttribute('name');
      }
      set value (value) {
        this.setAttribute('value', value);
      }
      attributeChanged(name, oldValue, newValue) {
        if ((name == 'checked') && (newValue != null))
          if (this.parentNode instanceof module.exports.RadioGroupElement)
            this.parentNode.setChecked(this);
      }
-   }).prototype,
+   },
  });
 </script>
 
 <!-- <radiogroup> -->
 <template id="radiogroup-shadow">
  <style>
   :host { padding: 1em; border: thin solid; }
  </style>
 </template>
 <script>
- module.exports.RadioGroupElement = module.registerElement({
+ module.exports.RadioGroupElement = module.registerElement{
    tagName: 'radiogroup',
    shadow: true,
-   prototype: (class extends Element {
+   constructor: (class extends Element {
      constructor (module) {
        super(module);
        this.shadowRoot.appendChild(module.document.findId('radiogroup-shadow').content.cloneNode(true));
      }
      get value () {
        let children = this.getChildNodes();
        for (let child of children)
          if (child instanceof module.exports.RadioElement)
            if (child.checked)
              return child.name;
        return '';
      }
      set value (name) { 
        let children = this.getChildNodes();
        for (let child of children)
          if (child instanceof module.exports.RadioElement)
            if (child.value == name)
              child.checked = true;
      }
      setChecked(radio) {
        if (!((radio instanceof module.exports.Radio) && radio.parentNode == this))
          throw;
        let children = this.getChildNodes();
        for (let child of children)
          if (child instanceof module.exports.RadioElement)
            if (child != radio)
              child.checked = false;
      }
-   }).prototype,
+   },
  });
 </script>