Dart test coverage not showing up for lib files

How do you make coverage reports include those in under lib directory? Right now when I run tests with dart test --coverage="coverage", it will only record coverage on my test files and not on the lib files. But when I change the imports declaration for my lib files relatively like:

import '../../lib/conventional.dart';

...instead of:

import 'package:conventional/conventional.dart';

...it works. Maybe I'm doing something wrong? Dart version is Dart SDK version: 2.12.0 (stable) on "linux_x64" and coverage version is 1.0.1. The source code can be found here: https://github.com/asartalo/conventional/