ExpansionPanel({
                             @required ExpansionPanelHeaderBuilder headerBuilder,
                              @required Widget body,
                              bool isExpanded: false
                         })

 
import 'package:flutter/material.dart';
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
        title: "Test",
        home: new Scaffold(
            appBar: new AppBar(
                title: new Text("Test"),
            ),
            body: new ListView(
                children: [
                  new ExpansionPanelList(
                    children: [
                      new ExpansionPanel(
                          headerBuilder: (BuildContext context, bool isExpanded) => const Text("Test"),
                          body: const Text("Test"),
                          isExpanded: true,
                      ),
                    ],
                  ),
                ],
            ),
        ),
    );
  }
}