Revert "Update typedef syntax to use Function notation and turn on lint for old notation. (#18035)" (#18041)

This reverts commit 32586020.

Fixes #18028

Just changes typedef declarations, no logic changes.

The new values are not ready for use yet inside Google

* Roll engine to e976be13c51448f89107d082ec81e2b6731671fa

* move away from deprecated constants

Fixes #13655

This updates AppContext per the recommendations in #15352

Fixes #15352

Also some trivial improvements to style.

Aggressively apply the const lint.

It's no longer needed with the latest Dart SDK roll.

* Make ProcessSignals portable

This removes the need to wrap unsupported signals with in `if (!platform.isWindows) ..`.

It also allows us to implement a work around for breaking the Windows console when flutter is exited with Ctrl+C.

* review comments

* adding tests

* add license header

This ensures that accidental usages of dart:io's
file API don't creep in over time.