Ensure that DevTools is installed.
Do you have DevTools installed?
If you are using an IDE,
make sure you have the Flutter and Dart plugins set up,
as described in the VS Code and
Android Studio and IntelliJ pages.
If you are working at the command line,
launch the DevTools server as explained in the
DevTools command line page.

Connect to DevTools.
When DevTools launches, you should see something
like the following:

    
Serving DevTools at http://127.0.0.1:9100
    
    Go to this URL in a Chrome browser. You should see the DevTools
launch screen. It should look like the following:

Connect to running app.
Under Connect to a running site,
paste the ws location that you copied in step 2,
and click Connect. You should now see Dart DevTools
running successfully in your Chrome browser:

    

    Congratulations, you are now running Dart DevTools!