Where is file core_esp8266_si2c.c for I2C fix?

Hi I’m struggeling with connecting a D1 Wemos (retired) board with a ProMini.
the D1 freezes from time to time. Now I found a possible fix here
http://www.digole.com/forum.php?topicID=777

but I cant find this file on my computer. Where is it located?

Those are my includes
#include <Arduino.h>
#include <ESP8266WiFi.h>
#include <ESP8266WiFiMulti.h>
#include <ESP8266HTTPClient.h>
#include <Wire.h>

Me neither. Is there something special about certain header files that they aren’t stored as files on the PC or is something else happening?

By the way my sketch compiles fine with #include <ESP8266WiFi.h>… I just can’t find it!!