72162 Make web buttons respond to enter key (cla: yes, f: material design, framework, waiting for tree to go green)

72163 Add some new examples to Actions and Shortcuts (cla: yes, f: material design, framework)

72169 Migrate the first batch of API samples to null safety (a: accessibility, cla: yes, framework, team, waiting for tree to go green)

72207 Fix navigator 2.0 in Flutter Web (cla: yes, framework, waiting for tree to go green)

72297 Migrate some material doc samples to null safety. (cla: yes, f: material design, framework, waiting for tree to go green)

72300 Fixed issue for SliverAppBar collapsedHeight (cla: yes, f: material design, f: scrolling, framework, waiting for tree to go green)