If the official Flutter source is too slow, you can switch the official source to some other sources. Just copy the following command to your Terminal.
vim ~/.bash_profile //open bash profile in vim editor export PUB_HOSTED_URL=https://pub.flutter-io.cn export FLUTTER_STORAGE_BASE_URL=https://storage.flutter-io.cn export PATH=/Users/your user name/flutter_dev/flutter/bin:$PATH

If you are using zsh

1 Open .zshrc in vim editor by inputting the following code vim $HOME/.zshrc 2 set environmental variable export PATH=$PATH:/Users/dandan/flutter/bin:$PATH 3 press ESC button on your keyboard and type wq! in your vim editor 4 type 'source .zshrc' in your Terminal