* apply upcoming lint cast_nullable_to_non_nullable

* make RenderObjectElement.renderObject non-nullable

Reland "Prevent viewport.showOnScreen from scrolling the viewport if the specified Rect is already visible. (#56413)" reverted in #64091 (#64513)

Revert "Prevent viewport.showOnScreen from scrolling the viewport if the specified Rect is already visible.  (#56413)" (#64091)

This reverts commit 64d76f2f.

Prevent viewport.showOnScreen from scrolling the viewport if the specified Rect is already visible.  (#56413)

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

This reverts commit 1d395c5e.

* Add missed Overflow

* 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

This reverts commit 2132a0c7.

This reverts commit 69080e07.

This normalizes all of the "See also" blocks in comments so that they conform in style.

They all look like this now:

  /// See also:
  ///
  ///  * [MyFavoriteWidget], because it's cool.
  class Foo {}

I removed some useless "See also" blocks, and added commentary to ones that were just "bare" references.

This is my penance for adding so many bullets that are "-" instead of "*". :-)

* implicit-casts:false in flutter/lib/src/rendering

* address review comments