81362 Add trailing commas in packages/flutter/test/widgets (framework, a: accessibility, 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)

81829 Revert “Migrate to ChannelBuffers.push” (a: tests, team, framework, f: material design, a: accessibility, cla: yes, f: cupertino, d: examples)

82057 Migrate to ChannelBuffers.push (a: tests, team, framework, f: material design, a: accessibility, cla: yes, f: cupertino, d: examples, waiting for tree to go green)