75813 Audit and update deprecation warnings (a: accessibility, cla: yes, f: material design, framework, team, waiting for tree to go green)

75873 Revert “Removed ButtonBar from AlertDialog” (cla: yes, f: material design, framework)

75899 Adding checkbox demo (cla: yes, f: material design, framework)

75903 deprecated consistency (cla: yes, f: material design, framework, team, waiting for tree to go green)

75937 [flutter_releases] Flutter Framework 1.26.0-17.6.pre Beta Cherrypicks (cla: yes, engine, f: material design, framework, tool)

75967 [TextSelectionTheme] improve docs (cla: yes, f: material design, framework, waiting for tree to go green)