82503 More toStrings and tests for physics, better docs (framework, cla: yes, waiting for tree to go green)

82508 Remove “unnecessary” imports. (a: tests, team, tool, framework, f: material design, cla: yes, waiting for tree to go green)

82525 Revert “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)

82533 Fix single frame image is decoded even cached (framework, cla: yes, waiting for tree to go green)

82539 Added Alignment docs for FractionallySizedBox (framework, cla: yes, waiting for tree to go green)