让NetworkImage进行下载

var ref = FirebaseStorage.instance.ref().child(fileName)
String location = await ref.getDownloadURL();
return new NetworkImage(downloadUrl);

更新

String _imageUrl;

void initState() {
  super.initState();

  var ref = FirebaseStorage.instance.ref().child(fileName)
  ref.getDownloadURL().then((loc) => setState(() => _imageUrl = loc));
}

...

        child: _imageUrl == null ? Image.asset('icons/default_profile_icon.png', height: 110.0,)
                :ImageNetwork(_imageUrl,fit: BoxFit.cover,