How do you make timeline without Intrinsic?

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget{

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Whatever'),
      ),
      body: _DetailListWidget(
        itemCount: 10,
        headContent: (int index) => '$index+++++',
        itemBuilder: (BuildContext context, int index) {
          return Text('得' * 18 * (index + 1));
        }
      ),
    );
  }
}

class _DetailListWidget extends StatelessWidget {
  const _DetailListWidget({
    Key key,
    @required this.headContent,
    @required this.itemCount,
    @required this.itemBuilder,
  }) : super(key: key);

  final String Function(int index) headContent;
  final int itemCount;
  final IndexedWidgetBuilder itemBuilder;

  Widget headerWidget(int index) {
    return Row(
      children: <Widget>[
        Image.network(
          'https://tva1.sinaimg.cn/large/007S8ZIlgy1giq9kstfyhj300r00r0mg.jpg',
          width: 10,
          height: 10,
        ),
        const SizedBox(width: 10),
        Text(
          headContent(index),
          style: TextStyle(
            color: Colors.grey,
            height: 1.35,
            fontSize: 13,
          ),
        ),
      ],
    );
  }

  Widget get dashBorder {
    return Positioned(
      top: 0.0,
      bottom: 0.0,
      left: 4.5,
      child: Image.network(
        'https://tva1.sinaimg.cn/large/007S8ZIlgy1giq9jawhn3j300300i02b.jpg',
        width: 1.25,
        repeat: ImageRepeat.repeatY,
      ),
    );
  }

  Widget contentContainer(BuildContext context, int index) {
    return Container(
      margin: EdgeInsets.symmetric(vertical: 12).copyWith(
        left: 19.5,
      ),
      padding: const EdgeInsets.symmetric(
        horizontal: 15,
        vertical: 10,
      ),
      width: double.maxFinite,
      decoration: BoxDecoration(
        borderRadius: BorderRadius.circular(5),
        color: Colors.grey[500],
      ),
      child: itemBuilder(context, index),
    );
  }

  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      padding: EdgeInsets.symmetric(vertical: 10),
      itemCount: itemCount,
      itemBuilder: (BuildContext context, int index) {
        return Padding(
          padding: EdgeInsets.symmetric(horizontal: 20),
          child: Column(
            mainAxisSize: MainAxisSize.min,
            children: <Widget>[
              headerWidget(index),
              Stack(
                children: <Widget>[
                  dashBorder,
                  contentContainer(context, index),
                ],
              ),
            ],
          ),
        );
      },
    );
  }
}