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

* Fix crash when widgetspan does not produce a semantics node in render paragraph

* fix comments

* fixed corner case, added test, refactored semantics widget

* addressing comment

My yearly pass to fix spelling errors in the API dartdocs.

When attempting to select a space on mobile, Flutter will try to select the word before the space too, if one exists.

Currently the framework handles delete, but not backspace, so embeddings
all have to implement backspace handling themselves. This eliminates
that inconsistency and allows simplified code in embeddings by adding
backspace handling.

It also fixes a bug uncovered in the delete handling where deleting a
selection would also delete the next character after the selection.

Move service extension to correct binding so images are repainted after enabling/disabling, update test (#68793)

* Move service extension to correct binding so images are repainted after enabling/disabling, update test

* Update binding.dart

Fixes https://github.com/flutter/flutter/issues/67343

Reland "Add CompositedTransformFollower.{followerAnchor, leaderAnchor} for custom anchoring (#64930)" (#65871) (#65884)

* enable null_check_on_nullable_type_parameter and tighten_type_of_initializing_formals

* fix CI

* Reland dispose images when done (#67100)

Changes since last time:

- Test for CanvasKit image rendering
  (https://github.com/flutter/flutter/pull/67176)
- Fix CanvasKit dispose impl
  (https://github.com/flutter/engine/pull/21555)
- Update internal google3 customer with a problematic ImageStream
  Listener impl (cl/335091311, cl/335459002)

This reverts commit 473358d9.

* Revert "docs for image disposal (#67066)"

This reverts commit bcb2ac5a.

* Revert "Dispose of images after using them (#66688)"

This reverts commit a7954691.

* Dispose of images after using them
Adds ImageInfo.clone, ImageInfo.dispose, and ImageInfo.isCloneOf.
Makes all stateful holders of ui.Image objects properly clone and dispose of them.
Behavior change for ImageStreamCompleter: Removing the last listener after registering at least one listener results in the object being disposed, and new listeners cannot be added, unless a keepAlive handle is created.
Makes all non-stateful holders of ui.Image (i.e. paintImage and RawImage) assert that whoever gave the image to them has not disposed it on them.

* apply upcoming lint cast_nullable_to_non_nullable

* make RenderObjectElement.renderObject non-nullable

Registering the service worker immediately after the documented has loaded may cause SW initialization to compete with framework initialization. It was recommended to us that we defer the service worker setup until after the framework is done with setup, which should be sometime after the first frame.

To implement this, I augmented the binding setup to dispatch an event on the document after the binding has initialized. I don't see any obvious risks with this setup.

Fixes #66066