77955 [flutter_tools] io cleanups to simplify null safety migration (cla: yes, tool, waiting for tree to go green)

77961 [flutter_tools] provide more context for invalidation (cla: yes, tool, waiting for tree to go green)

77962 Remove MockFlutterVersions (cla: yes, team, tool, waiting for tree to go green)

77968 Remove mocks from web devices test (cla: yes, team, tool, waiting for tree to go green)

77969 [flutter_tools] simplify shutdown hooks (cla: yes, tool, waiting for tree to go green)

77983 Remove MockBuildSystem from generate_synthetic_packages_test (cla: yes, team, tool, waiting for tree to go green)