Specs: default exports to {} rather than document, since it makes writing modules saner

Review URL: https://codereview.chromium.org/730223002

examples/htmlish/framework/element.sky

@@ -21,7 +21,7 @@ SKY MODULE - defines an <element> element
 <script>
  module.exports.Element = sky.registerElement({
    tagName: 'element',
-   prototype: class extends Element {
+   constructor: class extends Element {
      constructor (module) {
        super();
        this.state = 'loading';

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>