code: function(callback) {
        var template = this.fetchTemplate();
        var content = template && this.templateContent();
        if (content) {
          this.convertSheetsToStyles(content);
          var styles = this.findLoadableStyles(content);
          if (styles.length) {
            var templateUrl = template.ownerDocument.baseURI;
            return Polymer.styleResolver.loadStyles(styles, templateUrl, callback);
          }
        }
        if (callback) {
          callback();
        }
      }
docs: returns true if resources are loading