How to write a new custom lint rule in flutter or dart

All the logic for analysis of linter packages live in dart lang repo.
Specifically the following directory contains logic.

For eg avoid_empty_else is stored here

https://github.com/dart-lang/sdk/blob/9fa33d8acdff743fc7265d147abe9261deb149bc/pkg/linter/lib/src/rules/avoid_empty_else.dart#L27