67166 migrate material to nullsafety (a: tests, cla: yes, f: cupertino, f: material design, framework, waiting for tree to go green)

67167 Add missing margin to SnackBarAction (cla: yes, f: material design, framework, waiting for tree to go green)

67169 Make CupertinoTabView restorable (cla: yes, f: cupertino, framework, waiting for tree to go green)

67177 Reland dispose images when done (#67100) (a: tests, cla: yes, framework)

67183 Revert “migrate some material files to nullsafety” (cla: yes, f: material design, framework)