解决: 执行flutter run后出现Failed to download canvaskit
在命令行中输入：export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn
以使用国内镜像，如设置成功会出现：
Flutter assets will be downloaded from https://storage.flutter-io.cn. Make sure you trust this source!
在命令行中输入：export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn
以使用国内镜像，如设置成功会出现：
Flutter assets will be downloaded from https://storage.flutter-io.cn. Make sure you trust this source!