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)

72834 Check for iOS simulator SDK during AOT validation instead of assuming x86 (cla: yes, platform-ios, tool)

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)