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

@@ -1,10 +1,6 @@
 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>

specs/apis.md

@@ -311,7 +311,7 @@ module 'sky:core' {
     constructor (Application application, Document document, String url); // O(1)
     readonly attribute Application application; // O(1)
 
-    attribute any exports; // O(1) // defaults to the module's document
+    attribute any exports; // O(1) // defaults to {}
   }
 
   class Application : AbstractModule {

specs/modules.md

@@ -35,9 +35,10 @@ the [``Module`` object](apis.md) that represents the module.
 
 A module can export a value by assigning the ``exports`` property of
 its ``Module`` object. By default, the ``exports`` property of a
-``Module`` is its ``Document`` object, so that a script-less
-import is still useful (it exposes its contents, e.g. templates that
-the import might have been written to provide).
+``Module`` is an empty Object. Properties can be added to the object,
+or, it can be set to an entirely different object; for example, it
+could be set to the module's Document itself, in case the point of the
+module is to expose some ``template`` elements.
 
 ### Exporting element definitions ###