72558 Migrate some sample code to NNBD [1] (a: null-safety, cla: yes, d: api docs, d: examples, documentation, framework, team, waiting for tree to go green)

72755 Migrate some sample code to NNBD [2] (a: null-safety, cla: yes, d: api docs, d: examples, documentation, f: cupertino, framework, team, waiting for tree to go green)

72794 [NNBD] Migrate sample code pt 1 (a: null-safety, cla: yes, f: material design, framework, waiting for tree to go green)

72798 [NNBD] Migrate sample code pt 2 (a: null-safety, cla: yes, f: material design, framework, waiting for tree to go green)