Specs: Simplify the platform by only having one shadow tree per element.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/694613002

examples/radio.sky

@@ -11,11 +11,14 @@ SKY MODULE - radio button and radio button group
 </template>
 <script>
  module.exports = {};
- module.exports.RadioElement = sky.registerElement('radio', class extends Element {
+ module.exports.RadioElement = sky.registerElement({
+   tagName: 'radio',
+   shadow: true,
+   prototype: class extends Element {
      constructor () {
        super();
        this.addEventListener('click', (event) => this.checked = true);
-     this.addShadowRoot(new sky.ShadowRoot(module.document.findId('radio-shadow').content.cloneNode(true)));
+       this.shadowRoot.appendChild(module.document.findId('radio-shadow').content.cloneNode(true));
      }
      get checked () {
        return this.hasAttribute('checked');
@@ -37,6 +40,7 @@ SKY MODULE - radio button and radio button group
          if (this.parentNode instanceof module.exports.RadioGroupElement)
            this.parentNode.setChecked(this);
      }
+   },
  });
 </script>
 
@@ -47,10 +51,13 @@ SKY MODULE - radio button and radio button group
  </style>
 </template>
 <script>
- module.exports.RadioGroupElement = sky.registerElement('radiogroup', class extends Element {
+ module.exports.RadioGroupElement = sky.registerElement({
+   tagName: 'radiogroup',
+   shadow: true,
+   prototype: class extends Element {
      constructor () {
        super();
-     this.addShadowRoot(new sky.ShadowRoot(module.document.findId('radiogroup-shadow').content.cloneNode(true)));
+       this.shadowRoot.appendChild(module.document.findId('radiogroup-shadow').content.cloneNode(true));
      }
      get value () {
        let children = this.getChildNodes();
@@ -76,5 +83,6 @@ SKY MODULE - radio button and radio button group
            if (child != radio)
              child.checked = false;
      }
+   },
  });
 </script>