* 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