code: function isSpecialProperty(obj, key) {
    return isFunction(obj[key])
      || key.substring(0, 2) === '$$'
      || key.substring(0, 1) === '_';
  }
docs: An additional type checker used to determine if the property is of internal
  use or not a type that can be translated into JSON (like functions).
  
  @private
  @param {Object} obj The object which has the property to check
  @param {String} The property name to check
  @returns {Boolean} Whether the property is deemed special or not