Window is on the way out, we need to mark TestWindow and it's related properties as Deprecated to start driving people away from using them.

Proposal

• Update all flutter repo tests still using TestWindow to use TestPlatformDispatcher or TestFlutterView instead.
  • Update flutter_localizations/test to stop using TestWindow #122237
  • Update flutter/test/cupertino to no longer use TestWindow #122238
  • Update flutter/test/gestures to no longer reference TestWindow #122239
  • Update flutter/test/material to no longer use TestWindow #122240
  • Update flutter/test/rendering to no longer use TestWindow #122241
  • Update flutter/test/widgets to no longer reference TestWindow #122242
  • Update integration_test to no longer use TestWindow #122243
• Complete migration guide via: Migration guide for window singleton deprecation website#8283
• Mark TestWindow as deprecated.
  • Mark individual properties on TestWindow as deprecated as well and point users to the proper properties on TestPlatformDispatcher and TestFlutterView
  • Mark TestWidgetsFlutterBinding.window as deprecated and point users to WidgetTester.platformDispatcher and WidgetTester.view instead.