module.exports should default to an empty object.

Per the spec in modules.md the exports property should default to an
empty object. We lazy allocate it so that modules that just replace it
don't create the empty object and then throw it away.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/872043003

examples/city-list/city-list.sky

@@ -7,11 +7,6 @@
 <import src="city-data-service.sky" as="CityDataService" />
 <import src="city-sequence.sky" as="CitySequence" />
 
-<script>
-// TODO(esprehn): exports should start as the empty object.
-module.exports = {};
-</script>
-
 <sky-element name="state-header">
 <template>
   <style>