Have AndroidViewController extend PlatformViewController and add support for hybrid platform views (#60320)

Use Dart's characters package to fix user-facing grapheme cluster bugs.

This reverts commit 747fe345.

* Revert "Revert "Add clipBehavior to widgets with clipRect (#55977)" (#58344)"

This reverts commit 1d395c5e.

* Add missed Overflow

This reverts commit e0ed12c7.

* Word substitutions

* ++

* add language version 2.8 in packages/flutter

* enable non-nullable analyzer flag

This reverts commit cd593dae.

* Add clipBehavior to RenderFlex

* Add clipBehavior to FittedBox

* Add clipBehavior to Flex and FittedBox

* Add clipBehavior to UnconstrainedBox

* Add clipBehavior to Stack and Wrap

* Add clipBehavior to TextEditable

* Add clipBehavior to ListWheelScrollView

* Add clipBehavior to SingleChildScrollView

* Add clipBehavior to RenderViewportBase's widgets

Those widgets are NestedScrollView and ShrinkWrappingViewport.

* Fix tests

* Remove enum Overflow and fix typo

* Remove clipToSize

* Analyze fix

* Remove Mixin and other small fixes

* Fix tests and respect Stack's default clipBehavior

* Add Overflow back to make it non-breaking

* Restore clipBehavior to make it non-breaking

* Small fixes

* Fix rebase

Error message when size has not been set in RenderBox's performLayout should be well versed (#58151)

* Uses a special cursor value MouseCursor.defer to mark deferring. MouseTrackerAnnotation.cursor is now non-null. The mouseCursor argument of widgets or render objects can be null, which indicates using the default value.
* Moves SystemMouseCursors.uncontrolled to MouseCursor.uncontrolled.
* Changes how MouseCursor.debugDescription is defined. Previously MouseCursor.toString returns $runtimeType($debugDescription), while now it returns $debugDescription. Implementations of classes are updated, except for the ones of MouseCursor.defer and MouseCursor.uncontrolled are simply "defer" and "uncontrolled".

Adds the basic framework for the mouse cursor system. 

* Adds MouseRegion.cursor
* Adds SystemMouseCursors
* Adds mouseCursor to some widgets

The arrow key and delete handling in RenderEditable was using single
index values, which made it possible to move the cursor into the middle
of a surrogate pair (allowing things like adding text at that insertion
point), or to delete half of a surrogate pair. Since unpaired surrogate
pairs aren't valid UTF-16, doing so would cause assertions in the text
field.

This makes the arrow key and delete key handling surrogate-aware
(although not grapheme-cluster-aware, which is a larger fix that is out
of scope here).

Part of #55014

* disable hit testing if the CompositedTransformFollower is hidden when there is no leader layer

* update comment