code: function() {
    var formatted_stack_trace = UNDEFINED;
    var holder = this;
    while (holder) {
      var formatted_stack_trace =
        GET_PRIVATE(holder, formattedStackTraceSymbol);
      if (IS_UNDEFINED(formatted_stack_trace)) {
        // No formatted stack trace available.
        var stack_trace = GET_PRIVATE(holder, stackTraceSymbol);
        if (IS_UNDEFINED(stack_trace)) {
          // Neither formatted nor structured stack trace available.
          // Look further up the prototype chain.
          holder = %object_get_prototype_of(holder);
          continue;
        }
        formatted_stack_trace = FormatStackTrace(holder, stack_trace);
        SET_PRIVATE(holder, stackTraceSymbol, UNDEFINED);
        SET_PRIVATE(holder, formattedStackTraceSymbol, formatted_stack_trace);
      }
      return formatted_stack_trace;
    }
    return UNDEFINED;
  }
docs: Format the stack trace if not yet done, and return it. Cache the formatted stack trace on the holder.