Ensure that the engine frame callbacks are installed if the first scheduled frame is a forced frame (#101544)

See https://github.com/flutter/flutter/issues/98419

This reverts commit d056500b.

This reverts commit 31de052e.

This reverts commit e2490f29.

* Reschedule engine frame if it arrives in the middle of warm-up

* user post-frame callback instead

This reverts usages of the binding's platformDispatcher to use window again temporarily, because there isn't a TestPlatformDispatcher yet, and so some tests were failing because they mocked the TestWindow to return certain things (locales) that were returning the real values instead of the test values.

Once I've created a TestPlatformDispatcher to allow fake data to be passed to it, we can go back to using the platformDispatcher in all of these places

Remove references to `Window`, and switch usages to `PlatformDispatcher` or `SingletonFlutterWindow` (#69617)

* Remove references to dart:ui.Window, and point usages to PlatformDispatcher or SingletonFlutterWindow, as appropriate

* remove new test platform dispatchers

* Amend documentation

This reverts commit 2c7e5dd9.

This reverts commit d986fdc3.

* Implement stack trace demangling in the framework.

* Word substitutions

* ++

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag