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.
//open bash profile in vim editor
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
2 set environmental variable
3 press ESC button on your keyboard and type wq! in your vim editor
4 type ‘source .zshrc’ in your Terminal