Custom AppBar for Flutter

custom_appbar.dart

import 'package:flutter/material.dart';

class CustomAppBar extends StatelessWidget implements PreferredSizeWidget {

  final Widget title;
  final Widget leading;

  @override
  Size get preferredSize => new Size.fromHeight(kToolbarHeight);

  const CustomAppBar({Key key, this.title, this.leading}) : super(key: key);

  noSuchMethod(Invocation i) => super.noSuchMethod(i);

  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);

    return AppBar(
      leading: leading,
      titleSpacing: 0,
      title: title,
      centerTitle: false,
      elevation: 0,
      bottom: PreferredSize(
        preferredSize: Size.fromHeight(70),
        child: Container(
          color: theme.primaryColor,
          child: Padding(
            padding: EdgeInsets.all(0.0),
            child: Row(
              mainAxisAlignment: MainAxisAlignment.start,
              crossAxisAlignment: CrossAxisAlignment.start,
              children: <Widget>[
              ],
            ),
          ),
        ),
      ),
    );
  }
}

screen.dart

appBar: CustomAppBar(
  title: Text('Back'),
  leading: IconButton(
    icon: Icon(Icons.arrow_back_ios),
    onPressed: () => Navigator.of(context).maybePop(),
    tooltip: MaterialLocalizations.of(context).backButtonTooltip,
  )
)