appBarGradient.dart

import 'package:flutter/material.dart';

class AppBarGradient extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      decoration: BoxDecoration(
        gradient: LinearGradient(
          begin: Alignment.topLeft,
          end: Alignment.bottomRight,
          colors: <Color>[
            Color(0xFFff9966),
            Color(0xFFff5e62)
          ],
          tileMode: TileMode.clamp
        )
      ),
    );
  }
}

// Uso
AppBar(
  title: Text('AppBar Gradient'),
  flexibleSpace: AppBarGradient(),
),