67916 Flutter Driver: command extensions and extension feature cleanup (a: tests, cla: yes, framework, waiting for tree to go green)

67946 Migrate Scaffold SnackBars in the framework to ScaffoldMessenger (a: tests, cla: yes, d: api docs, d: examples, documentation, f: material design, framework, team, waiting for tree to go green)

67952 Replace obsolete FlatButton reference in flutter_driver extension_test.dart (a: tests, cla: yes, framework)

68019 Set slider semantics flag for sliders (a: tests, cla: yes, f: cupertino, f: material design, framework, waiting for tree to go green)

68032 [flutter_test] export fake from flutter_test (a: tests, cla: yes, framework)