63595 Disables tests failing due to new semantics flag. (a: accessibility, a: tests, cla: yes, framework, waiting for tree to go green)

63609 Clean up flutter_gallery__transition_perf (a: accessibility, cla: yes, team, waiting for tree to go green)

63799 bug fix for flutter_gallery__transition_perf(_with_semantics) (a: accessibility, cla: yes, team)

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

64120 [flutter_tools] reland: enable –null-assertions for flutter framework tests (a: accessibility, cla: yes, framework, team, waiting for tree to go green)

64672 Nnbd widgets (a: accessibility, cla: yes, f: cupertino, framework, team)