it('can test for network error', () => {
  const emsg = 'simulated network error';

  httpClient.get(testUrl).subscribe(
    data => fail('should have failed with the network error'),
    (error: HttpErrorResponse) => {
      expect(error.error.message).toEqual(emsg, 'message');
    }
  );

  const req = httpTestingController.expectOne(testUrl);

  // Create mock ErrorEvent, raised when something goes wrong at the network level.
  // Connection timeout, DNS error, offline, etc
  const mockError = new ErrorEvent('Network error', {
    message: emsg,
  });

  // Respond with mock error
  req.error(mockError);
});