This is follow-up clean-up for #26193
Co-authored-by: NHunter Freyer <hjfreyer@google.com>

* Add embedder unit test that reproduces the problem described in
https://github.com/dart-lang/sdk/issues/46275

* Address code review comments.

https://dart.googlesource.com/sdk.git/+log/b410651bd18e..f82b36d0b4f0

2021-07-20 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-337.0.dev
2021-07-20 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-336.0.dev
2021-07-19 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-335.0.dev
2021-07-19 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-334.0.dev
2021-07-19 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-333.0.dev

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dart-sdk-flutter-engine
Please CC garyq@google.com,dart-vm-team@google.com,zra@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

Extract the prebuilt Dart SDK to a temp directory and then move it after the extraction completes (#27569)

* Extract the prebuilt Dart SDK to a temp directory and then move it after the extraction completes

This ensures that the GN script will not see an invalid Dart SDK at the
expected path if the extract fails.

* Add os_arch to temp extraction path
Co-authored-by: NZachary Anderson <zanderso@users.noreply.github.com>