[material_ui] Remove widgets/ import in flexible_space_bar_test.dart, app_bar_sliver_test.dart and partial fix in chip_test.dart - #12004

Dashboard Checks is a set of presubmit jobs to ensure nothing breaks when changes are landed. For details on individual job executions, please see the presubmit dashboard via the link below. It becomes green automatically when all jobs successfully complete. If it fails, you can view failure details, get execution logs, and re-run failed jobs on the presubmit dashboard page. If you suspect that this check is not working correctly, contact #hackers-infra on Discord. If you need to merge your PR without presubmit checks (a rare situation, typically an emergency), then you can use the emergency label.