docs: function mapBy(dependentKey, propertyKey) {
      _emberMetalDebug.assert('Ember.computed.mapBy expects a property string for its second argument, ' + 'perhaps you meant to use "map"', typeof propertyKey === 'string');
  
      return map(dependentKey + '.@each.' + propertyKey, function (item) {
        return _emberMetalProperty_get.get(item, propertyKey);
      });
    }