70014 [flutter_tools] remove workaround for caching sound dill (a: null-safety, cla: yes, team, tool)

70714 [flutter_tools] use frontend_server for web test compilation (a: null-safety, cla: yes, team, tool)

70718 [flutter_tools] display message for current null safety mode (a: null-safety, cla: yes, tool)

71807 StandardMethodCodec.decodeEnvelope should allow null error message (a: null-safety, cla: yes, framework)

72476 Allow nullable tweens in TweenAnimationBuilder (a: animation, a: null-safety, cla: yes, framework, waiting for tree to go green)