67360 Migrate more tests to null safety (cla: yes, framework, waiting for tree to go green)

67361 Characters docs (cla: yes, f: cupertino, f: material design, framework, team, waiting for tree to go green)

67364 Make FlutterErrorDetails.exception non-nullable as documented (cla: yes, framework, waiting for tree to go green)

67375 Add rootOverlay flag to [Draggable], to put feedback on root [Overlay] (cla: yes, framework)

67410 Fix the tree (cla: yes, f: cupertino, framework)

67414 Add tristate to parent checkbox for DataTable (cla: yes, f: material design, framework)