52879 Allow recent Dart language features (cla: yes, tool)

52891 [flutter_tools] refactor iOS install/uninstall to new file (cla: yes, tool)

52893 Revert “[flutter_tools] refactor GenSnapshot and AotBuilder” (cla: yes, tool)

52894 [Material] Allow Appbar to exclude header semantics (a: accessibility, cla: yes, f: material design, framework, waiting for tree to go green)

52896 [flutter_tools] refactor AotBuilder, removing globals and duplicated kernel compilation (cla: yes, tool)

52900 Roll engine 216c420a2c06..373ebe6430b2 (2 commits) (cla: yes, waiting for tree to go green)