The loadEntrypoint function calls the onEntrypointLoaded callback
once the Service Worker is initialized, and the main.dart.js entrypoint
has been downloaded and run by the browser. Flutter also calls
onEntrypointLoaded on every hot restart during development.

The onEntrypointLoaded callback receives an engine initializer object as
its only parameter. Use the engine initializer to set the run-time
configuration, and start the Flutter Web engine.

The initializeEngine() function returns a Promise
that resolves with an app runner object. The app runner has a
single method, runApp(), that runs the Flutter app.

Customizing web app initialization

In this section,
learn how to customize each stage of your app’s initialization.

Loading the entrypoint

The loadEntrypoint method accepts these parameters:

entrypointUrl

The URL of your Flutter app’s entrypoint. Defaults to "main.dart.js".

String