Angular 4.x: No provider for Service

I am having trouble injecting a service into another service in Angular 4.x and receive the error: Error: No provider for SkillsService!

I have created a repo that reproduces this error. You can run this locally by cloning the repo and simply running ng test from the repo root directory.

Steps i took...

1. Create app with ng new
2. Create ContactService with ng g service contact
3. Create SkillsService with ng g service skills
4. Add SkillsService to constructor of ContactService (with @Inject annotation)
5. Add both SkillsService and ContactService to app.module.ts as providers
6. Run ng test and receive the error: Error: No provider for SkillsService!

How do I add a provider to ContactService for the SkillsService?

It seems like it must be something very simple, yet it's proving hard to work out from the documentation and searching around.