JavascriptChannel on iOS

I use JavascriptChannel to pass a string back to the app form webview.
It works perfectly for Android but can't make it work on iOS? Is there anything specific I need to do to make it work?

```
class WebViewJavascriptChannel {
static Set getChannel(Function(String) handler) {
return [
JavascriptChannel(
name: 'JavascriptChannel',
onMessageReceived: (JavascriptMessage javascriptMessage) {
handler(javascriptMessage.message);
print(javascriptMessage.message);
}),
].toSet();
}
}

Widget _getBankIDWebView(String url) {
return WebviewScaffold(
javascriptChannels:
WebViewJavascriptChannel.getChannel(_updateString),
url: url,
withZoom: false,
hidden: true,
appBar: _getAppBar(),
);
}

```

从 WebviewScaffold 和 issue 中所示的 javascriptChannels 配置开始，然后在 iOS 上重现消息流，并将其与 Android 进行比较。完成的标准是：issue 记录了已确认的 iOS 特定要求，或记录了一个可复现的失败及范围明确的修复和验证步骤。

由索引模型根据 Issue 内容生成。