69596 Updated the button.icon factory constructors for NNBD (cla: yes, f: material design, framework)

69610 Make header optional in PaginatedDataTable (cla: yes, f: material design, framework, waiting for tree to go green)

69617 Remove references to Window, and switch usages to PlatformDispatcher or SingletonFlutterWindow (a: accessibility, a: tests, cla: yes, f: material design, framework, team, tool)

69620 Remove deprecated methods from BuildContext (cla: yes, f: cupertino, f: material design, framework, severe: API break, waiting for tree to go green)

69631 Revert TextField.adaptive (cla: yes, f: material design, framework)

69650 Update ReorderableListView API docs (cla: yes, f: material design, framework)