This reverts commit 4cc325fa.

Issue #2835604 by Wim Leers, effulgentsia: BigPipe provides functionality, not an API: mark all classes & interfaces @internal

(cherry picked from commit 074f9fcb36b6e7be266c01aa718437e344860e84)

Issue #2567715 by marthinal, hgoto, jpd4nt, mr.baileys, Wim Leers, alexpott: #markup generated by #pre_render callbacks do not get processed by ensureMarkupIsSafe()

(cherry picked from commit 397b680a)

Issue #2792817 by Wim Leers, dawehner: BigPipe decorates html_response.attachments_processor, but uses an inner name based on the decorated service, which can cause problems

(cherry picked from commit b9aa30a0)

Issue #2712935 by Wim Leers, Fabianx, catch, alexpott: Messages are not rendered last, meaning those messages set within placeholders only appear on the next request

This reverts commit 6418a1c5.

Issue #2678662 by Wim Leers, mondrake, Fabianx, alexpott: Ensure BigPipe does not break when HTML document contains CDATA sections or inline scripts matching certain patterns

Issue #2752413 by danmuzyka, alexpott, jhedstrom: Custom CSS classes added to status messages print on page as content when using Classy theme or subthemes

Issue #2723705 by Wim Leers, catch: Reduce the 200ms BigPipe placeholder interval to 50ms and make it overridable

Issue #2698811 by danmuzyka, Wim Leers, jhedstrom, Fabianx: BigPipe unnecessarily renders and sends multiple-occurrence placeholders multiple times when using JS — can cause JS errors

Issue #2719695 by alexpott, Mile23: Fix 'Drupal.Classes.ClassDeclaration.CloseBraceAfterBody' coding standard

Issue #2572777 by andypost, alexpott, attiks, andriyun: Fix 'Squiz.Strings.ConcatenationSpacing' coding standard

Issue #2702609 by Wim Leers, Berdir, Fabianx: Disable placeholdering (and BigPipe) on unsafe requests to help find forms as fast as possible (to allow EnforcedResponses to work)

Issue #2702001 by olafkarsten, Wim Leers: Inject config factory for BigPipe (including in exception handlers)

Issue #2702537 by ifrik, Wim Leers: Update the BigPipe module description to be in line with the Help text standard

Issue #2665992 by alexpott, klausi, heykarthikwithu, xjm, tstoeckler: @file is not required for classes, interfaces and traits

Issue #2678568 by Wim Leers: Ensure good UX & DX even when A) rendering of placeholder fails, B) some response event subscriber fails

Revert "Issue #2678568 by Wim Leers: Ensure good UX & DX even when A) rendering of placeholder fails, B) some response event subscriber fails"

This reverts commit eee35007.

Issue #2678568 by Wim Leers: Ensure good UX & DX even when A) rendering of placeholder fails, B) some response event subscriber fails

Issue #2670444 by Wim Leers: Embedded AJAX responses aren't application/json, but application/vnd.drupal-ajax

Issue #2679867 by Wim Leers, mr.baileys: BigPipe causes PHP notices with JavaScript disabled on error pages

Issue #2678628 by Wim Leers: Consider removing the BigPipe no-JS cookie when JS is enabled again in a browser

Issue #2469431 by Wim Leers, Fabianx, swentel, nod_, yched, dawehner, tstoeckler, catch, borisson_: BigPipe for auth users: first send+render the cheap parts of the page, then the expensive parts