Flutter 根据不同 Flavor 配置不同 Assets 资源

这样可以避免把不同的 flavor 资源都打入包内，减小包体积

flutter:
  assets:
    - assets/common/
    - path: assets/flavor1/
      flavors:
        - flavor1
    - path: assets/flavor2/
      flavors:
        - flavor2

参见：docs.flutter.dev/tools/pubsp…