Trying to mock an angular provider. unsuccessfully

I've been trying to mock a provider (for use in another provider) and couldn't manage to get it mocked. In the test, I see the actual provider function and not the mocked one.

Here is the (simplified) code:

First, the setup: (I removed anything that isn't needed):

function ServiceToMock() {
  this.functionMock = function () {
    // do something...
  };
  this.$get = function () {
    // doesn't matter
  };
}
module('theModule').provider('serviceToMock', ServiceToMock);

function ServiceThatUsesMock(serviceToMock) {

  serviceToMock.functionMock(); // I want the mocked function to run here but the actual one does

  this.$get = function () {
    // doesn't matter...
  };
}
module('theModule').provider('serviceThatUsesMock', ServiceThatUsesMock);

As you can see, two services defined with .provider(), the second is dependent on the first.

And the unit test:

let functionMock = jasmine.createSpy();
module('theModule');
module(function ($provide) {
  $provide.provider('serviceToMock', function () {
    this.functionMock = functionMock;
    this.$get = () => ({});
    this.test = 'aaa';
  });
});

var serviceThatUsesMock;
beforeEach(inject(function (_serviceThatUsesMock_) {
  serviceThatUsesMock = _serviceThatUsesMock_;
}));

it('should register for update failure', function () {
  expect(functionMock).toHaveBeenCalled();
});

The test fails because the real function gets called and not the mocked one.

What Am I Doing wrong?

Thanks!