65568 Remove unused ‘dart:async’ imports. (a: accessibility, a: internationalization, a: tests, cla: yes, f: cupertino, f: material design, framework, team, tool)

65658 Make Navigator restorable (inkl. WidgetsApp, MaterialApp, CupertinoApp) (cla: yes, f: cupertino, f: material design, framework, waiting for tree to go green)

65880 migrate some cupertino files to nullsafety (cla: yes, f: cupertino, framework, waiting for tree to go green)

66020 Remove deprecated activity indicator (cla: yes, f: cupertino, framework, waiting for tree to go green)

66024 migrate some cupertino files to nullsafety (cla: yes, f: cupertino, framework)