83443 Clean up null assumptions for Gradle classes (tool, cla: yes, waiting for tree to go green, a: null-safety)

83445 Clean up null assumptions for project.dart dependencies (tool, cla: yes, waiting for tree to go green, a: null-safety)

83504 Remove more mocks from error_handling_io and attach_test (team, tool, cla: yes, waiting for tree to go green, a: null-safety)

83817 Migrate project.dart and all dependencies to null safety (team, tool, cla: yes, a: null-safety)

83854 Migrate deferred_components_prebuild_validator to null safety (tool, cla: yes, waiting for tree to go green, a: null-safety)