8249 Allow specifying an alternate Mac host SDK. (cla: yes)

8250 Delegate a11y events and action to/from embedded Android platform views. (cla: yes)

8251 Fixed service isolate not being initialized correctly due to bad name (cla: yes)

8253 Add platformViewId to stub_ui (cla: yes)

8254 Do not drop the DartExecutor’s message handler when a FlutterNativeView is detached from the FlutterView (cla: yes)

8256 check that public API of dart:ui conforms to web implementation (cla: yes)

8265 Linking Higher & Lower Class Docs (affects: docs, cla: yes)

8273 Allow embedders to specify their own task runner interfaces. (cla: yes)