Flutter not picking custom fonts based on fontWeight

From the docs we get this list of constants:

w100 Thin, the least thick
w200 Extra-light
w300 Light
w400 Normal / regular / plain
w500 Medium
w600 Semi-bold
w700 Bold
w800 Extra-bold
w900 Black, the most thick

So in pubspec you can define your custom font like this:

  fonts:
   - family: Montserrat
     fonts:
       - asset: fonts/Montserrat-Regular.ttf
       - asset: fonts/Montserrat-SemiBold.ttf
         weight: 600
       - asset: fonts/Montserrat-Bold.ttf
         weight: 700

and use it in your code like this:

final h1 = new TextStyle(
    fontSize: 24.0, 
    fontWeight: FontWeight.w600  // semi-bold
);