3550 [file_selector_web] Migrated to null-safety (cla: yes, nnbd, p: file_selector, platform-web)

3554 [file_selector_web] update documentation #76067 (cla: yes, p: file_selector, platform-web)

3605 [file_selector_platform_interface] null safety stable release (cla: yes, nnbd, p: file_selector)

3630 [file_selector] Return a non-null value from getSavePath on web (cla: yes, p: file_selector, platform-web, waiting for tree to go green)

3631 [file_selector] Migrate to null safety (cla: yes, nnbd, p: file_selector)

3643 [file_selector] Endorse web (cla: yes, p: file_selector)