66455 apply upcoming lint cast_nullable_to_non_nullable (a: tests, cla: yes, framework)

66482 TextSelectionTheme support (step 3 of 3) (cla: yes, f: material design, framework, team)

66493 migrate cupertino to nullsafety (cla: yes, f: cupertino, framework, waiting for tree to go green)

66500 Flutter 1.22.0-12.2.pre Framework cherrypicks (a: tests, cla: yes, engine, framework, team, tool)

66504 Re-land ScaffoldMessenger (a: quality, cla: yes, customer: money (g3), f: material design, framework, severe: new feature, waiting for tree to go green)