66849 Flutter 1.22.0-12.3.pre framework cherrypicks (a: tests, cla: yes, engine, f: cupertino, f: material design, framework, team, tool)

66914 Move assert(s) that reference ‘this’ to the constructor bodies. (cla: yes, f: cupertino, f: material design, framework)

67020 Relax the bounds of some Cupertino text field tests (cla: yes, f: cupertino, framework)

67085 Migrate some tests to null-safety (cla: yes, f: cupertino, framework, waiting for tree to go green)

67086 [NNBD] Migrate some Cupertino tests (cla: yes, f: cupertino, framework, waiting for tree to go green)