68133 Migrate Switch tests to null safety. (cla: yes, f: material design, framework, waiting for tree to go green)

68135 [flutter_releases] Flutter 1.22.2 framework cherrypicks (a: tests, cla: yes, engine, f: cupertino, f: material design, framework, team, tool)

68136 Sync lints (cla: yes, f: cupertino, f: material design, framework, team, tool, waiting for tree to go green)

68150 Migrate some widget tests to NNBD (a: accessibility, cla: yes, framework, waiting for tree to go green)

68151 Updated Builder API doc (cla: yes, framework, waiting for tree to go green)