* Revert "Revert "Improve performance of collectAllElements (#68065)" (#68207)"

This reverts commit 46ff57d6.

* use fewer elements for benchmark

[devicelab] allow the devicelab to take a screenshot if the iOS connection fails with FLUTTER_IOS_SCREENSHOT_ON_CONNECTION_FAILURE (#68156)

More attempts to remote diagnose the issues in the iOS devicelab

Migrate yet even more widget framework tests to null safety.

This reverts commit d962c8f2.

This crash affects anyone using flutter, so move it into the tool where it can be tested and documented. Ensure it works on windows devicelab too.

This reverts commit 6aa8447a.

This reverts commit 8ba5732c.

* NNBD widget test migration

These androidx migration tests require downloading firebase, which is a large library with a number of dependencies - this causes frequent enough flakes.

AndroidX migration is ~2 years in the past, and while it would be nice to confirm this still works, ideally that would be done in a reduced manner.

* [flutter_tools] partial revert of start app change

- run_without_leak_tests: have been disabled for months
- build_benchmark: the tooling work here is mostly done and we're not tracking further improvements, free up more devicelab capacity
- system_debug_ios: does not work post iOS13
- mac_enable_twc: not adding more mac tests to devicelab
- hello_world_start_up: disabled

Any tests that we think will be valuable in the future can be resurrected from the git history.

Revert "[flutter_tools] If run fails to install the application on iOS, try uninstalling it and trying again. (#68078)" (#68118)

This reverts commit 61ba7dd2.