9045 remove over-optimistic assert (cla: yes)

9049 New Plugin API PR4: Adds Lifecycle support to the new plugin system. (cla: yes)

9054 Add mouse button support to the macOS shell (cla: yes)

9058 libtxt: have GetRectsForRange(strut) fall back to tight bounds if layout isn’t forcing use of the strut (cla: yes)

9060 Add missing top level to stub_ui (cla: yes)

9061 [scene_host] Cleanup scene_host closures (cla: yes)

9062 Add a podspec for FlutterMacOS.framework (cla: yes)

9072 Roll third_party/dart/tools/sdks to 2.3.0 (cla: yes)