lib/main.dart (MyApp)

Note:
  The Material library implements widgets that follow Material
  Design principles. When designing your UI, you can exclusively use
  widgets from the standard widgets library, or you can use
  widgets from the Material library. You can mix widgets from both
  libraries, you can customize existing widgets,
  or you can build your own set of custom widgets.

Non-Material apps

For a non-Material app, you can add the Center widget to the app’s
build() method:

lib/main.dart (MyApp)

By default a non-Material app doesn’t include an AppBar, title,
or background color. If you want these features in a non-Material app,
you have to build them yourself. This app changes the background
color to white and the text to dark grey to mimic a Material app.

That’s it! When you run the app, you should see Hello World.