MonoScript from AssetImporter?

I tried to get the path of .cs file where the AssetImporter class code is written, but could not find the way.

If the script is about MonoBehaviour or ScriptableObject, we can find MonoScript object, and get its path from AssetDatabase.

In the case of AssetImporter scripts, what function is available?

I could not find an importer for scripts, but found the ScriptableImporter class that lets you create importers for your own extensions. It seems available from 2018.1, but is experimental.