'flutter' is not recognized as an internal or external command,

1) Make sure that the environment variables are set correctly,

Go to “Control Panel > User Accounts > User Accounts > Change my environment 
variables”

Under “User variables” check if there is an entry called “Path”:

If the entry does exist, append the full path to flutter\bin using ; as a 
separator from existing values.

If the entry does not exist, create a new user variable named Path with the 
full path to flutter\bin as its value.

2) Reboot your computer

Once both above steps are done, run flutter doctor

Note: Run this command in either a Command Prompt or PowerShell window. 
Currently, Flutter does not support third-party shells like Git Bash. (Source: 
Flutter Documentation)