I am stumped. I checked the source both in the SDK folder and github and no sign of it. From other answers it is clear to me that they want to hide that from the public because of the changes that might happen in that or any internal class for that matter. So, my question shouldn't be interpreted around, "Why do I need to access that in the first place?"
I can go by without looking at that code but the question is that when the R class is not available in the sources, how come Android Studio is compiling the code referencing it?
Also, there is no library in Java where you can't ultimately peek the source code so how has Google managed to do it?
For that matter, I also tried to import a fictitious package and class and it didn't compile as expected.