iOS 11.4 simulator crash

Just want to try this plugin but have:

```
MissingPluginException(No implementation found for method close on channel flutter_webview_plugin)
#0 MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:278:7)

#1 FlutterWebviewPlugin.close (package:flutter_webview_plugin/src/base.dart:121:30)
#2 _WebviewScaffoldState.initState (package:flutter_webview_plugin/src/webview_scaffold.dart:51:22)
#3 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3751:58)
#4 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3617:5)
#5 Element.inflateWidget (package:flutter/src/widgets/framework.dart:2907:14)
#6 Element.updateChild (package:flutter/src/widgets/framework.dart:2710:12)
#7 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3653:16)
#8 Element.rebuild (package:flutter/src/widgets/framework.dart:3495:5)
#9 ComponentElement._firstBuild (package:<…>
[VERBOSE-2:dart_error.cc(16)] Unhandled exception:
MissingPluginException(No implementation found for method launch on channel flutter_webview_plugin)
#0 MethodChannel.invokeMethod (package:flutter/src/services/platform_channel.dart:278:7)

#1 FlutterWebviewPlugin.launch (package:flutter_webview_plugin/src/base.dart:110:20)

#2 _WebviewScaffoldState.build (package:flutter_webview_plugin/src/webview_scaffold.dart:64:24)
#3 StatefulElement.build (package:flutter/src/widgets/framework.dart:3730:27)
#4 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3642:15)
#5 Element.rebuild (package:flutter/src/widgets/framework.dart:3495:5)
#6 ComponentElement._firstBuild (package:flutter/src/widgets/framework.dart:3622:5)
#7 StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:3769:11)
#8 ComponentElement.mount (package:flutter/src/widgets/framework.dart:3617:5)
#9 Elemen<…>
```

Works on my Android device.

在 iOS 11.4 模拟器上重现该报告，并将其与正常工作的 Android 情况进行比较。从 src/base.dart 的第 110-121 行以及 src/webview_scaffold.dart 的第 51 和 64 行开始，然后跟踪 flutter_webview_plugin 的 channel 调用。当模拟器不再针对 close 或 launch 报告 MissingPluginException 时，即表示完成。

由索引模型根据 Issue 内容生成。