Upgraded Scaffold example to an application example.

* Revert "Make host app test actually test host app (#24131)"

This reverts commit 27293857.

* Revert "Roll engine 5646e86a6f44..2584816d78a3 (2 commits) (#24197)"

This reverts commit 846a5eea.

* Make host app test actually test host app

* mark updated test as flaky

https://github.com/flutter/engine/compare/5646e86a6f44...2584816d78a3

git log 5646e86a6f44..2584816d78a3 --no-merges --oneline
2584816d7 Roll Dart version to f9ebf2129732fd2b606286fdf58e500384b8a0bc (flutter/engine#6812)
9669aeb95 Roll src/third_party/skia 06a477c3300d..ef4709b7850d (31 commits) (flutter/engine#6811)


The AutoRoll server is located here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

* Re-order platform_integration to be more featured
* Remove flutter_goldens_client
* Fix snippet precompilation to always run `pub get` and
  to detect snapshot failure

To see if it stops the Cirrus crashing

Adds a note mentioning that this is still a release preview.

https://github.com/flutter/engine/compare/316f6365a64b...5646e86a6f44

git log 316f6365a64b..5646e86a6f44 --no-merges --oneline
5646e86a6 Use the internal_nodes_canvas for all leaf node operations. (flutter/engine#6804)
21c265cce Synchronize Flutter's rendering with CA. (flutter/engine#6807)
a64049a14 Update Vulkan headers to 1.1 (flutter/engine#6806)
f5e5d81ee Move the embedded view preview flag check to a common function. (flutter/engine#6813)


The AutoRoll server is located here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

* fixes wheel gradient and adds test