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)

70112 [null-safety] migrate hello_world to null safety (cla: yes, d: examples, team)

70184 [FloatingActionButtonLocation] Add diagrams to documentation of FloatingActionButtonLocation (cla: yes, d: api docs, d: examples, documentation, f: material design, framework, waiting for tree to go green)

70224 Move Flutter.framework to build directory instead of ios/Flutter (cla: yes, d: examples, platform-ios, t: xcode, team, tool)