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

77956 [devicelab] Migrate Gallery to BuildTestTask (a: accessibility, cla: yes, team, 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)