7 files declare their use of StreamedResponse in Zircon Profile 8

ClientTest.php in vendor/symfony/http-kernel/Tests/ClientTest.php

DiactorosFactory.php in vendor/symfony/psr-http-message-bridge/Factory/DiactorosFactory.php

DiactorosFactoryTest.php in vendor/symfony/psr-http-message-bridge/Tests/Factory/DiactorosFactoryTest.php

FragmentHandler.php in vendor/symfony/http-kernel/Fragment/FragmentHandler.php

PageCache.php in core/modules/page_cache/src/StackMiddleware/PageCache.php

Contains \Drupal\page_cache\StackMiddleware\PageCache.

StreamedResponseListener.php in vendor/symfony/http-kernel/EventListener/StreamedResponseListener.php

StreamedResponseTest.php in vendor/symfony/http-foundation/Tests/StreamedResponseTest.php