Flutter如何制作像gmail一样的浮动动作按钮动画？

我能够像Gmail应用程序一样制作非常相似的浮动动作按钮动画，但是当我isExpanded为 false时，我会获得一点余量。有什么解决办法吗？

这是我的代码

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 StatefulWidget {
@override
_MyHomePageState createState() => _MyHomePageState();
}
class _MyHomePageState extends State {
bool isExpanded = false;
Widget build(context) {
return Scaffold(
floatingActionButton: AnimatedContainer(
width: isExpanded ? 150 : 56,
height: 56,
duration: Duration(milliseconds: 300),
child: FloatingActionButton.extended(
onPressed: () {},
icon: Icon(Icons.ac_unit),
label: isExpanded ? Text("Start chat") : SizedBox(),
),
),
appBar: AppBar(),
body: FlatButton(
onPressed: () {
setState(() {
isExpanded = !isExpanded;
});
},
child: Text('Press here to change FAB')));
}
}