PLUS: clean-up of all the unreachable stuff.

Re-land of https://github.com/flutter/flutter/pull/123297 without any of
the commits at the end attempting to fix customer-testing.

Fixes https://github.com/flutter/flutter/issues/118810

Revert "[flutter_tools] Remove --no-sound-null-safety #3"

[flutter_tools] Remove --no-sound-null-safety #3

Revert "[flutter_tools] Remove sound null safety flag (#123031)"

[flutter_tools] Remove sound null safety flag #2

This reverts commit 7c3088cf.

[flutter_tools] Remove sound null safety flag

* Roll flutter engine to a24c7c13925e4e3282f7b85814b70e63782fa057

* migrate some material files to nullsafety

* fix test_private

* address review comments

* fix private test on windows

* use uri in include entry of analysis_options