72303 Migrate some more material doc samples to null safety. (cla: yes, f: material design, framework, waiting for tree to go green)

72305 InteractiveViewer NNBD Docs Migration (cla: yes, framework)

72307 RawAutocomplete NNBD Docs Migration (cla: yes, framework)

72308 Add ScrollbarTheme/ScrollbarThemeData (cla: yes, f: material design, f: scrolling, framework, severe: new feature, waiting for tree to go green)

72316 Remove Scrollbar.isAlwaysShown assert based on Scrollbar.controller (a: quality, cla: yes, f: cupertino, f: material design, f: scrolling, framework, severe: new feature, waiting for tree to go green)