3487 Automatically add platform labels (cla: yes)

3490 Remove iOS stubs (cla: yes, p: android_alarm_manager, p: android_intent, p: connectivity, p: espresso, p: file_selector, p: flutter_plugin_android_lifecycle, p: google_maps_flutter, p: google_sign_in, p: image_picker, p: integration_test, p: path_provider, p: shared_preferences, p: url_launcher, p: video_player, platform-ios, platform-linux, platform-macos, platform-web, platform-windows)

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

3492 [image_picker_platform_interface] migrate to nnbd  (cla: yes, nnbd, p: image_picker, waiting for tree to go green)