64050 1.21.0-9.1.pre cherrypicks (cla: yes, engine, f: material design, framework, team, tool)

64055 migrate semantics to nullsafety (a: accessibility, cla: yes, framework, waiting for tree to go green)

64062 Revert “Prevent material switch from recreating its render object when it becomes disabled” (cla: yes, f: material design, framework, waiting for tree to go green)

64071 [null-safety] enable null assertions for framework tests (cla: yes, framework, team, waiting for tree to go green)

64076 remove ununsed deprecated field (cla: yes, framework, waiting for tree to go green)

64091 Revert “Prevent viewport.showOnScreen from scrolling the viewport if the specified Rect is already visible. “ (cla: yes, f: material design, framework)