67165 [flutter_tools] update build rules to depend on subset of package_config contents (cla: yes, tool)

67166 migrate material to nullsafety (a: tests, cla: yes, f: cupertino, f: material design, framework, waiting for tree to go green)

67167 Add missing margin to SnackBarAction (cla: yes, f: material design, framework, waiting for tree to go green)

67169 Make CupertinoTabView restorable (cla: yes, f: cupertino, framework, waiting for tree to go green)

67171 [null-safety] add integration tests for sound null safety modes, add support for sound null safety in dart2js (a: null-safety, cla: yes, team, tool, waiting for tree to go green)