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

that uses it looks nicer; this moves the complexity to places that
register elements without using 'class' (e.g. the internals of
frameworks that create elements for defining elements)

Review URL: https://codereview.chromium.org/699083005

examples/radio.sky

@@ -16,7 +16,7 @@ SKY MODULE - radio button and radio button group
  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);
@@ -42,7 +42,7 @@ SKY MODULE - radio button and radio button group
          if (this.parentNode instanceof module.exports.RadioGroupElement)
            this.parentNode.setChecked(this);
      }
-   }).prototype,
+   },
  });
 </script>
 
@@ -53,10 +53,10 @@ SKY MODULE - radio button and radio button group
  </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));
@@ -85,6 +85,6 @@ SKY MODULE - radio button and radio button group
            if (child != radio)
              child.checked = false;
      }
-   }).prototype,
+   },
  });
 </script>