auto.$provide.service()

Register a service constructor, which will be invoked with new to create the service instance. This is short for registering a service where its provider's $get property is a factory function that returns an instance instantiated by the injector from the service constructor function.

Internally it looks a bit like this:

{
  $get: function() {
    return $injector.instantiate(constructor);
  }
}

You should use $provide.service(class) if you define your service as a type/class.