72754 [flutter_releases] Flutter Beta 1.25.0-8.2 Framework Cherrypicks (cla: yes, engine, framework, team, tool)

72755 Migrate some sample code to NNBD [2] (a: null-safety, cla: yes, d: api docs, d: examples, documentation, f: cupertino, framework, team, waiting for tree to go green)

72766 Migrated some cupertino doc comments to null safety. (cla: yes, f: cupertino, framework, waiting for tree to go green)

72772 Unprefixes the class with the “new” keyword (cla: yes, f: material design, framework, waiting for tree to go green)