docs: function generateAssets(generator, output) {
      var assets = output.getAssets();
      var logger = output.getLogger();
  
      // Is generator ignoring assets?
      if (!generator.onAsset) {
          return Promise(output);
      }
  
      return Promise.reduce(assets, function(out, assetFile) {
          logger.debug.ln('copy asset "' + assetFile + '"');
  
          return generator.onAsset(out, assetFile);
      }, output);
  }