67585 Add documentation talking about ScrollPhysics.applyTo(null) (cla: yes, d: api docs, documentation, framework, waiting for tree to go green)

67940 Fix NestedScrollView sample code (a: quality, cla: yes, d: api docs, d: examples, documentation, framework, waiting for tree to go green)

67946 Migrate Scaffold SnackBars in the framework to ScaffoldMessenger (a: tests, cla: yes, d: api docs, d: examples, documentation, f: material design, framework, team, waiting for tree to go green)

69781 Added dartpad examples for SliverAppBar (adopt a widget, cla: yes, d: api docs, d: examples, documentation, f: material design, framework, team, waiting for tree to go green)