public function requireOnceThrowsExceptionForInvalidIdentifier($identifier)
{
$mockCache = $this->createMock(AbstractFrontend::class);
$mockCache->expects($this->atLeastOnce())->method('getIdentifier')->will($this->returnValue('UnitTestCache'));
$backend = $this->prepareDefaultBackend();
$backend->setCache($mockCache);
$backend->requireOnce($identifier);
}