I made a Flutter package that might help with this question called fxf.

To create "Hello World", you can do the following:

import 'package:fxf/fxf.dart' as fxf;

class MyWidget extends StatelessWidget {
  ...
  Widget build(BuildContext context) {
    return Center(
      child: fxf.Text("Hello *(5)World!"),
    );
  }
}

... where *(5) is a style command that bolds any text written after it to FontWeight.w900.