code: def is_builtin_function(id_node):  # type: (Dict[str, Any]) -> bool
      
      # Builtin functions are always IDENTIFIER.
      if NodeType(id_node['type']) is not NodeType.IDENTIFIER:
          return False
  
      id_value = id_node['value']
  
      if not is_function_identifier(id_node):
          return False
  
      # There are difference between a function identifier and variable
      # identifier:
      #
      #   let localtime = 0
      #   echo localtime " => 0
      #   echo localtime() " => 1420011455
      return id_value in BuiltinFunctions
docs: Whether the specified node is a builtin function name identifier.
      The given identifier should be a child node of NodeType.CALL.