81529 fix a MaterialApp NNBD issue (framework, f: material design, cla: yes, waiting for tree to go green)

81578 Enable library_private_types_in_public_api lint (a: tests, team, tool, framework, f: material design, a: accessibility, cla: yes, f: cupertino, d: examples, waiting for tree to go green)

81604 prototypeItem added to ReorderableList and ReorderableListView (framework, f: material design, cla: yes)

81631 Clean up some old and obsolete TODOs of mine (framework, f: material design, cla: yes, waiting for tree to go green)

81634 Change elevation to double of MergeableMaterial (framework, f: material design, cla: yes, waiting for tree to go green)