3491 Remove Dart stubs from macOS plugins (cla: yes, p: connectivity, p: path_provider)

3498 [path_provider] Update macOS for NNBD (cla: yes, nnbd, p: path_provider, platform-macos)

3513 Update to ffi 0.3.0-nullsafety.1 (cla: yes, p: path_provider, platform-windows)