code: function(value) {
  			var attr = this.get('class');
  			value = utils.trim(value);
  			if (!attr) {
  				return this.add('class', value);
  			}
  
  			var classVal = attr.value();
  			var classList = ' ' + classVal.replace(/\n/g, ' ') + ' ';
  			if (!~classList.indexOf(' ' + value + ' ')) {
  				attr.value(classVal + ' ' + value);
  			}
  		}
docs: A special case of attribute editing: adds class value to existing
  `class` attribute
  @param {String} value