Это то, что вы ищете?
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Widget build(BuildContext context) {
return SafeArea(
child: Scaffold(
body: Column(
children: <Widget>[
Container(
height: 70,
alignment: Alignment.center,
color: Colors.red,
child: Text(
"ADVERTISE HERE!",
style: TextStyle(color: Colors.white, fontWeight: FontWeight.bold, fontSize: 24),
),
),
AppBar(title: Text("Main page")),
],
),
),
);
}