fromRequestStack example

$alias_manager->expects($this->any())
      ->method('getAliasByPath')
      ->willReturnCallback([$this, 'aliasManagerCallback']);

    $this->aliasManager = $alias_manager;

    $this->requestStack = new RequestStack();
    $request = Request::create('/some/path');
    $this->requestStack->push($request);

    $this->context = new RequestContext();
    $this->context->fromRequestStack($this->requestStack);

    $processor = new AliasPathProcessor($this->aliasManager);
    $processor_manager = new PathProcessorManager();
    $processor_manager->addOutbound($processor, 1000);
    $this->processorManager = $processor_manager;

    $this->routeProcessorManager = $this->getMockBuilder('Drupal\Core\RouteProcessor\RouteProcessorManager')
      ->disableOriginalConstructor()
      ->getMock();

    $generator = new UrlGenerator($this->provider, $processor_manager, $this->routeProcessorManager, $this->requestStack, ['http', 'https']);