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

66851 EditableText action handlers swallow errors (cla: yes, framework)

66858 migrate some material files to nullsafty (cla: yes, f: material design, framework)

66864 More EditableText docs (cla: yes, framework, waiting for tree to go green)

66889 Add more unit test cases for EditableText widget (cla: yes, framework, waiting for tree to go green)

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