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.

Reproduce the report on an iOS 11.4 simulator and compare it with the working Android case. Start at src/base.dart lines 110-121 and src/webview_scaffold.dart lines 51 and 64, then trace the flutter_webview_plugin channel calls. Done means the simulator no longer reports MissingPluginException for close or launch.

Written by the indexing model from the issue text.