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

83384 Mark flutter_gallery__transition_perf_e2e flaky (team, cla: yes, team: flakes)

83407 enable lint prefer_interpolation_to_compose_strings (a: tests, team, tool, framework, f: material design, a: accessibility, a: internationalization, cla: yes, f: cupertino, d: examples, waiting for tree to go green)

83433 fix lint from an improved unnecessary_parenthesis (team, tool, framework, cla: yes, waiting for tree to go green)

83439 Modify key info generation for new iOS key code. (team, cla: yes, waiting for tree to go green)