Flutter 调试工具报错：No active package devtools.

命令行启动 devtools : flutter pub global run devtools 报错信息如下：

No active package devtools.
pub finished with exit code 65

出现这样的错误是没有安装并激活调试工具，通过以下命令就可以激活 devtools：

flutter pub global activate devtools

MacOS 安装的时候会出现以下警告，让配置环境变量，如果需要就配置，不需要则不用管他：

Warning: Pub installs executables into $HOME/.pub-cache/bin, which is not on your path.
You can fix that by adding this to your shell's config file (.bashrc, .bash_profile, etc.):

  export PATH="$PATH":"$HOME/.pub-cache/bin"