M3E typography Emily's original code

snippet.dart

factory M3ETypography.defaultFor(
    Brightness brightness, {
    String? brandFontFamily,
    String? plainFontFamily,
    TextTheme? baseOverride,
  }) {
    // 1) Start from Material 3 baseline type.
    final m3 = Typography.material2021();
    final TextTheme m3Base =
        baseOverride ?? (brightness == Brightness.dark ? m3.white : m3.black);

    // 2) Optionally map brand/plain families to role groups (M3E guidance).
    final TextTheme baseWithFamilies = _applyFamilies(
      m3Base,
      brand: brandFontFamily,
      plain: plainFontFamily,
    );

    // 3) Provide emphasized deltas (weights/tracking).
    return M3ETypography(
      base: baseWithFamilies,
      emphasized: M3EEmphasized.forBrightness(brightness),
    );
  }