window.Print.postMessage is undefined

## System info

Issue occurs on: both
Plugin version: 0.3.11
Flutter doctor output:

```
[✓] Flutter (Channel unknown, v1.12.13+hotfix.9, on Linux, locale en_US.UTF-8)
• Flutter version 1.12.13+hotfix.9 at /media/user/disk3/Applications/flutter
• Framework revision f139b11009 (3 months ago), 2020-03-30 13:57:30 -0700
• Engine revision af51afceb8
• Dart version 2.7.2

[✓] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
• Android SDK at /media/user/disk3/Android/Sdk
• Android NDK location not configured (optional; useful for native profiling support)
• Platform android-29, build-tools 28.0.3
• ANDROID_HOME = /media/user/disk3/Android/Sdk
• ANDROID_SDK_ROOT = /media/user/disk3/Android/Sdk
• Java binary at: /snap/android-studio/90/android-studio/jre/bin/java
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
• All Android licenses accepted.

[✓] Android Studio (version 4.0)
• Android Studio at /snap/android-studio/90/android-studio
• Flutter plugin version 45.1.1
• Dart plugin version 193.7361
• Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

[✓] Android Studio (version 3.6)
• Android Studio at /snap/android-studio/88/android-studio
• Flutter plugin version 44.0.2
• Dart plugin version 192.7761
• Java version OpenJDK Runtime Environment (build 1.8.0_212-release-1586-b4-5784211)

[✓] Connected device (1 available)
• Redmi 5 Plus • 0d559abd9905 • android-arm64 • Android 8.1.0 (API 27)
```

Hi,

I have a problem at loaded html page. My javascript code is here

```js
if (window.Print !== undefined) {
window.Print.postMessage(JSON.stringify({
event: 'polygonCreated',
coordinates: currentCoordinates.map(function (currentCoordinate) {
return [currentCoordinate.lat, currentCoordinate.lng];
}),
}));
} else {
alert('Error');
}
```

type of window.Print is object but I cant access Print.postMessage at release mode. It's working in debug mode but at release mode doesn't work

## Logs

I dont have any logs because no error

まず、報告された window.Print.postMessage 呼び出しを使って、読み込まれた HTML ページを release モードで再現し、その後、上記に記載された Flutter プラグインのバージョンと Android 環境を使って debug モードと比較します。release モードで bridge が異なる理由を特定し、そこで postMessage が動作することを確認するか、制限事項を文書化できれば完了です。

索引モデルが issue の本文から書いたものです。