[Bug]Strange number of HTTP status code

## System info

Issue occurs on: iOS
Plugin version: ^0.3.11
Flutter doctor output:

```
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel unknown, 1.18.0-11.1.pre, on Mac OS X 10.15.6 19G2021,
locale ja-JP)

[!] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
! Some Android licenses not accepted. To resolve this, run: flutter doctor
--android-licenses
[✓] Xcode - develop for iOS and macOS (Xcode 11.7)
[✓] Chrome - develop for the web
[!] Android Studio
✗ Flutter plugin not installed; this adds Flutter specific functionality.
✗ Dart plugin not installed; this adds Dart specific functionality.
[✓] Android Studio (version 4.0)
[✓] VS Code (version 1.49.1)
[✓] Connected device (3 available)

! Doctor found issues in 2 categories.
```

I have 2 Android Studios. And one is not setup yet but not used. so don't worry about it.

## Steps to Reproduce

1. login yahoo
2. status code `-999`
`WebViewHttpError`
`error.code` is -999
3. what is this number??

do you know what is HTTP status code is?

## Logs

`-999`

iOS で Yahoo のログインフローを再現し、error.code が -999 に設定されて発生する WebViewHttpError を調査する。その値が HTTP ステータスなのかネイティブ WebView エラーなのかを判断し、期待される報告動作を定義する。

索引モデルが issue の本文から書いたものです。