59128 Fix typo in scroll_aware_image_provider.dart (cla: yes, framework, waiting for tree to go green)

59156 Add sample code to PositionedTransition (cla: yes, framework)

59162 Rebuild SliverAppBar when forceElevated changes (cla: yes, f: material design, framework, waiting for tree to go green)

59186 Opt out nnbd in packages/flutter (a: accessibility, cla: yes, f: cupertino, f: material design, framework)

59187 Support floating the header slivers of a NestedScrollView (a: annoyance, a: quality, cla: yes, customer: crowd, customer: quill (g3), d: api docs, d: examples, documentation, f: material design, f: scrolling, framework, waiting for tree to go green)