A bug in _RenderCustomClip was compaeing the type of oldClipper to
itself instead to the type of newClipper.

This was the root cause for the crash #14937 worked around.
This also reverts the workaround introduced in #14937.

Minor typo fix for BottomAppBar doc

The downcast was crashing when toggling hasNotch for BottomAppBar.

The Floating Action Button adds some margin around it for the notch.
So we need to compute a notch even when the FAB and the Bottom App Bar
doe not overlap.

* add a FAB NotchMaker to ScaffoldGeometry

* add a notchMaker to FloatingActionButton

* Initial implementation of BottomAppBar

Mainly includes the notch making logic.
Not yet tested as currently there is no way to make the FAB and the BAB
overlap, once #14368 to lands we could add unit tests to the
BottomAppBar as well.

* use a closeable for clearing the FAB notchmaker