Instrumented file https://gist.github.com/scheglov/4aed80727855aafd627cd4137dfeeea7

flutter_outline_live_preview_instrumented.dart

import 'package:flutter/material.dart';

class MyWidget extends StatelessWidget {
  final String firstText;

  MyWidget(this.firstText);

  factory MyWidget.forDesignTime() {
    return _registerWidgetInstance(0, new MyWidget('AA'));
  }

  @override
  Widget build(BuildContext context) {
    return _registerWidgetInstance(1, new Column(
      crossAxisAlignment: CrossAxisAlignment.center,
      children: <Widget>[
        _registerWidgetInstance(2, buildText(firstText)),
        _registerWidgetInstance(3, buildText('BBBB')),
        _registerWidgetInstance(4, buildText('CCCCCC')),
        _registerWidgetInstance(5, buildText('DDDDDDDD')),
      ],
    ));
  }

  Text buildText(String text) {
    return _registerWidgetInstance(6, new Text(text, style: const TextStyle(fontSize: 24.0)));
  }
}
final flutterDesignerWidgets = <int, Widget>{};

T _registerWidgetInstance<T extends Widget>(int id, T widget) {
  flutterDesignerWidgets[id] = widget;
  return widget;
}