MixedViewport didn't use the building:true flag when locking itself, so
when it caused a rebuild of its children, we assumed that nobody was
allowed to mark things dirty below the list, and things crashed when
Inherited people did in fact rebuild.
Also:
- default offset for MixedViewport
- don't bother rebuilding if the underlying RenderObject is going to
rebuild anyway for some reason
- better docs for the "items must have keys" assert
- keep the FlipComponent stuff together in test_widgets.dart
c1de88233e