56258 Roll engine 906bf5968465..2037e0f18d9b (1 commits) (waiting for tree to go green)

56274 Roll engine 2037e0f18d9b..180a497ee558 (5 commits) (cla: yes, waiting for tree to go green)

56290 Turn implicit casts in code generated by flutter_platform.dart into explicit casts. (cla: yes, waiting for tree to go green)

56295 Fix overly specific detection of non-UTF8 files in analyzer bot. (cla: yes)

56309 Trial PR to use an engine that has the null safety unfork CL in it. (cla: yes)

56324 add macOS to ui integration test (cla: yes)