istanbul mocha coverage still shows code as not executed

foo.js

process.on('exit', () => { // Runs, but coverage report marks these lines as not executed!
  log.info('Start !!!');
  c.close();
  b.close();
  a.stop();
  log.info('Exit !!!');
});

fooTest.js

describe('process', () => {
  context('exit', () => {
    let save, spy;

    before(() => {
      save = sinon.stub(process, 'on').withArgs('exit');
      spy = sinon.spy(log, 'info');
    });

    after(() => {
      log.info.restore();
      process.on.restore();
    });

    it('must exit', () => {
      save.yields(process.emit('exit'));
      expect(spy.callCount).to.be.equal(2);
    });
  });
});