72837 [NNBD] Migrate sample code pt 3 (a: null-safety, cla: yes, f: material design, framework, team, waiting for tree to go green)

72890 Remove deprecated Scaffold.resizeToAvoidBottomPadding (cla: yes, f: material design, framework, severe: API break, waiting for tree to go green)

72903 Remove deprecated Element methods (cla: yes, f: cupertino, f: material design, framework, severe: API break, waiting for tree to go green)

72938 [NNBD] Migrate sample code pt 6 (cla: yes, f: material design, framework)

72939 Complete migration to null safety in api documentation (a: null-safety, cla: yes, f: cupertino, f: material design, framework, team)