Cupertino sliding segmented control is translucent

main.dart

import 'package:flutter/cupertino.dart';

void main() {
  runApp(
    CupertinoApp(
      theme: CupertinoThemeData(
        brightness: Brightness.light,
      ),
      home: CupertinoPageScaffold(
        child: Stack(
          children: <Widget>[
            Text('Hello world'),
            CupertinoSlidingSegmentedControl<int>(
              onValueChanged: (value) {},
              children: {
                0: Text('Foo'),
                1: Text('Bar'),
              }
            ),
          ],
        ),
      ),
    ),
  );
}