code: function defaultAssets( { name, category, descriptor } ) {
     switch( category ) {
        case 'themes':
           return {
              assetUrls: [ descriptor.styleSource || 'css/theme.css' ]
           };
        case 'layouts':
        case 'widgets':
        case 'controls':
           return {
              assetsForTheme: [ descriptor.templateSource || `${name}.html` ],
              assetUrlsForTheme: [ descriptor.styleSource || `css/${name}.css` ]
           };
        default:
           return {};
     }
  }
docs: Return the default assets for the given artifact, determined by it's type
  and descriptor's `styleSource` and `templateSource` attributes.
  @param {Object} artifact an artifact created by the {@link ArtifactCollector}
  @return {Object} a partial descriptor containing the artifact's default assets