Run ng build --prod and then navigate to the folder ./dist/ng-cli-safari-issue. Serve this folder over http using your favorite http server, for example using the npm packages http-serve or live-serve. Open the site on Safari and inside the Web Inspector window look at the Network tab about the JS files being loaded. As you see the ES5 as well as the ES2015 bundles have been loaded altogether as shown by the screenshot below:
One possible way to remedy this behaviour under Safari seems to be through through this commit. That's duplicating all the script tags inside a given index.html using the type=module and nomodule attributes.
This project was generated with Angular CLI version 8.1.1.
Run ng serve for a dev server. Navigate to http://localhost:4200/. The app will automatically reload if you change any of the source files.
Run ng generate component component-name to generate a new component. You can also use ng generate directive|pipe|service|class|guard|interface|enum|module.
Run ng build to build the project. The build artifacts will be stored in the dist/ directory. Use the --prod flag for a production build.
Run ng test to execute the unit tests via Karma.
Run ng e2e to execute the end-to-end tests via Protractor.
To get more help on the Angular CLI use ng help or go check out the Angular CLI README.