83310 Migrate localizations and generate_synthetic_packages to null safety (team, tool, cla: yes, waiting for tree to go green, a: null-safety)

83311 Migrate deferred_components_gen_snapshot_validator to null safety (team, tool, cla: yes, waiting for tree to go green, a: null-safety)

83381 Migrate build system build.dart to null safety (team, tool, cla: yes, waiting for tree to go green, a: null-safety)

83441 Migrate pubspec_schema to null safety (tool, cla: yes, waiting for tree to go green, a: null-safety)

83442 Clean up null assumptions for Xcode and CocoaPods classes (tool, cla: yes, waiting for tree to go green, a: null-safety)