code: @Override
    public final Map<String, JSType> getPropertyTypeMap() {
      Map<String, JSType> propTypeMap = new LinkedHashMap<>();
      updatePropertyTypeMap(this, propTypeMap, new HashSet<FunctionType>());
      return propTypeMap;
    }
docs: get the map of properties to types covered in a function type
  
  @return a Map that maps the property's name to the property's type