To setup your development environment run:
git clone git@github.com:angular/angular
cd devtools
yarn
To run the extension in development mode run:
ng serve
This would start a development server that you can access on http://localhost:4200. In development, Angular DevTools uses a "development shell." This is different from "chrome shell" in a way, that it runs the user's app in an iframe. DevTools then communicate with the user's app via message passing.
To build the Angular DevTools Chrome extension run:
npm run release
This would output the production assets of the app in dist/shell-browser. If you want to build the extension with development settings (i.e., not minified sources), just run:
ng build shell-browser
Later on, you can install the extension from dist/shell-browser following the guide from here.