title: '1 required argument(s) expected but 0 found' error
body: <p>I'm trying my hand at building an app with flutter (which uses Dart 2). I'm getting an error which I cant figure out:</p>
  
  <p>The error is at the <code>child: new Center(</code></p>
  
  <pre><code>@override
  Widget build(BuildContext context) {
      return new Scaffold(
        appBar: new AppBar(
          title: new Text('Begin with a search...'),
          actions: &lt;Widget&gt; [
            new FlatButton(
              child: new Text('Logout', style: new TextStyle(fontSize: 17.0, color: Colors.white)),
              onPressed: _signOut,
            )
          ]
        ),
        body: new Container(
          margin: EdgeInsets.symmetric(horizontal: 20.0),
          child: new Center(
            child: new Row(
              children: &lt;Widget&gt;[
                new TextField(
                  controller: _controller,
                  autofocus: true,
                  textAlign: TextAlign.start,
                  decoration: new InputDecoration(
                    prefixIcon: new Padding(
                      padding: const EdgeInsetsDirectional.only(end: 10.0),
                      child: new Icon(Icons.search),
                    ),
                    border: new OutlineInputBorder(borderRadius: new BorderRadius.all(Radius.circular(20.0))),
                    hintText: 'Search for a recipient',
                  ),
                  onChanged: null,
                ),
                new FlatButton(
                  onPressed: () {_controller.clear();},
                  child: new Icon(Icons.clear)
                ),
              ],
            ),
          ),
        ),
        bottomNavigationBar: NavBar(),
      );
    } 
  </code></pre>