I am running on web and getting this error. Error: MissingPluginException(No implementation found for method close on channel flutter_webview_plugin)

## System info

Issue occurs on: WEB
Plugin version: latest
Flutter doctor output:

Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 2.0.0, on macOS 11.2.2 20D80 darwin-x64, locale en-PK)
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
[✓] Xcode - develop for iOS and macOS
[✓] Chrome - develop for the web
[✓] Android Studio (version 4.1)
[✓] Connected device (1 available)

• No issues found!

## Steps to Reproduce
Simply just run it on web and the page doesn't load and through an exception.

## Logs

[+4371 ms] Error: MissingPluginException(No implementation found for method close on channel flutter_webview_plugin)
at Object.throw_ [as throw] (http://localhost:53868/dart_sdk.js:5334:11)
at MethodChannel._invokeMethod (http://localhost:53868/packages/flutter/src/services/system_channels.dart.lib.js:954:21)
at _invokeMethod.next ()
at http://localhost:53868/dart_sdk.js:39032:33
at _RootZone.runUnary (http://localhost:53868/dart_sdk.js:38889:58)
at _FutureListener.thenAwait.handleValue (http://localhost:53868/dart_sdk.js:33875:29)
at handleValueCallback (http://localhost:53868/dart_sdk.js:34435:49)
at Function._propagateToListeners (http://localhost:53868/dart_sdk.js:34473:17)
at _Future.new.[_completeWithValue] (http://localhost:53868/dart_sdk.js:34315:23)
at async._AsyncCallbackEntry.new.callback (http://localhost:53868/dart_sdk.js:34338:35)
at Object._microtaskLoop (http://localhost:53868/dart_sdk.js:39176:13)
at _startMicrotaskLoop (http://localhost:53868/dart_sdk.js:39182:13)
at http://localhost:53868/dart_sdk.js:34689:9
[ +6 ms] Error: MissingPluginException(No implementation found for method launch on channel flutter_webview_plugin)
at Object.throw_ [as throw] (http://localhost:53868/dart_sdk.js:5334:11)
at MethodChannel._invokeMethod (http://localhost:53868/packages/flutter/src/services/system_channels.dart.lib.js:954:21)
at _invokeMethod.next ()
at http://localhost:53868/dart_sdk.js:39032:33
at _RootZone.runUnary (http://localhost:53868/dart_sdk.js:38889:58)
at _FutureListener.thenAwait.handleValue (http://localhost:53868/dart_sdk.js:33875:29)
at handleValueCallback (http://localhost:53868/dart_sdk.js:34435:49)
at Function._propagateToListeners (http://localhost:53868/dart_sdk.js:34473:17)
at _Future.new.[_completeWithValue] (http://localhost:53868/dart_sdk.js:34315:23)
at async._AsyncCallbackEntry.new.callback (http://localhost:53868/dart_sdk.js:34338:35)
at Object._microtaskLoop (http://localhost:53868/dart_sdk.js:39176:13)
at _startMicrotaskLoop (http://localhost:53868/dart_sdk.js:39182:13)
at http://localhost:53868/dart_sdk.js:34689:9

まず Flutter 2.0.0 の Web で失敗を再現し、そのプラットフォームで plugin が close メソッドと launch メソッドをどのように処理しているかを調べます。報告された Web ページが MissingPluginException エラーなしで読み込まれ、必要なプラットフォーム動作が定義され、適切なテストでカバーされていれば完了です。

索引モデルが issue の本文から書いたものです。