Reland "Prevent material switch from recreating its render object when it becomes disabled (#61398)", reverted in #64062 (#64354)

On a local benchmark, removing compute is faster than compute in both debug and release mode on a Pixel 4. On a MotoG4, a much larger text sample (800 Kb) takes 50 ms, so we cannot simple remove the limit. Increase to 50 Kb which should take at most a ms or two on an older device, and only microseconds on newerones.

Bug:
  https://github.com/flutter/flutter/issues/64070

More work towards removal of mockito to facilitate strong null safety tests.

#62886

The remaining uses of this pattern were all due to wanting to have
the child's slot when `Element.forgetChild()` was called. However,
when that method is called, the child's `slot` value is still valid
in the context of the parent, so the uses can just use `child.slot`.

This is the final round of cleanup from the fallout of #63269

Revert "Prevent material switch from recreating its render object when it becomes disabled (#61398)" (#64062)

* Minor adjustment for engine change, and rev template version

* whitespace fix

This wires up the new WindowProc delegation system that allows plugins
to handle top-level window messages (e.g., to control resize behavior).

Fixes #53168

* add run_if property for existing try builders

* update

* update readme

This reverts commit ce40de69.

(this re-applies #64189 by reverting #64249)

This reverts commit 21cfed34.

* Redesigns the interface between MouseTracker and RendererBinding&RenderView.
* Simplifies the structure of RenderMouseRegion.
* Extracts the common utility code between mouse_tracker_test and mouse_tracker_cursor_test.