75863 Clean up some documentation around shader warm-up. (cla: yes, framework, waiting for tree to go green)

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

75894 Fix FocusManager constructor (a: tests, cla: yes, framework)

75895 Refactor Hero example (cla: yes, framework, waiting for tree to go green)

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)