code: function extendAssets( {
     assets = [],
     assetUrls = [],
     assetsForTheme = [],
     assetUrlsForTheme = []
  } = {}, source ) {
     return {
        assets: assets.concat( source.assets ),
        assetUrls: assetUrls.concat( source.assetUrls ),
        assetsForTheme: assetsForTheme.concat( source.assetsForTheme ),
        assetUrlsForTheme: assetUrlsForTheme.concat( source.assetUrlsForTheme )
     };
  }
docs: ///////////////////////////////////////////////////////////////////////////////////////////////////////////