72829 Migrate some sample code to NNBD [3] (a: null-safety, cla: yes, d: api docs, d: examples, documentation, f: cupertino, framework, team, waiting for tree to go green)

72833 Fix scrollbar configuration for hover events (a: desktop, cla: yes, f: material design, f: scrolling, framework, platform-web, waiting for tree to go green)

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

72838 Migrated some more widgets doc comments to null safety. (cla: yes, framework, waiting for tree to go green)

72842 [NNBD] Migrate sample code pt 4 (a: null-safety, cla: yes, framework, waiting for tree to go green)