[printing] No Windows desktop support

Describe the bug

On Windows, the Printing.layoutPdf util does not show up the printing dialog.

It does not even show any error/exception.

To Reproduce

In a button callback and with an existing assets/files/document.pdf file.

print('Printing...');

try {
    final pdf = await rootBundle.load(
        'assets/files/document.pdf',
    );

    print('File size: ${pdf.lengthInBytes}');

    final printed = await Printing.layoutPdf(
      onLayout: (_) => pdf.buffer.asUint8List(),
    );

    print('Printed: $printed');
} catch (e) {
    print(e.runtimeType);
}

Expected behavior

The printing dialog is expected to be shown or, at least, an identifiable exception should be thrown.

Screenshots

No error message and no dialog:

Flutter Doctor

Flutter Doctor output

``` [√] Flutter (Channel stable, 2.0.3, on Microsoft Windows [Version 10.0.19041.985], locale en-US) • Flutter version 2.0.3 at C:\flutter • Framework revision 4d7946a68d (9 weeks ago), 2021-03-18 17:24:33 -0700 • Engine revision 3459eb2436 • Dart version 2.12.2 [√] Android toolchain - develop for Android devices (Android SDK version 30.0.2) • Android SDK at C:\Users\karlo\AppData\Local\Android\sdk • Platform android-30, build-tools 30.0.2 • Java binary at: C:\Program Files\Android\Android Studio\jre\bin\java • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01) • All Android licenses accepted. [√] Chrome - develop for the web • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe [√] Visual Studio - develop for Windows (Visual Studio Community 2019 16.9.6) • Visual Studio at C:\Program Files (x86)\Microsoft Visual Studio\2019\Community • Visual Studio Community 2019 version 16.9.31313.79 • Windows 10 SDK version 10.0.19041.0 [√] Android Studio (version 4.1.0) • Android Studio at C:\Program Files\Android\Android Studio • Flutter plugin can be installed from: https://plugins.jetbrains.com/plugin/9212-flutter • Dart plugin can be installed from: https://plugins.jetbrains.com/plugin/6351-dart • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b01) [√] VS Code (version 1.56.2) • VS Code at C:\Users\karlo\AppData\Local\Programs\Microsoft VS Code • Flutter extension version 3.22.0 [√] Connected device (3 available) • Windows (desktop) • windows • windows-x64 • Microsoft Windows [Version 10.0.19041.985] • Chrome (web) • chrome • web-javascript • Google Chrome 90.0.4430.212 • Edge (web) • edge • web-javascript • Microsoft Edge 90.0.818.62 • No issues found! ```

Platform:

[ ] iOS
[ ] Android
[ ] Browser
[x] Windows 10 - HUAWEI MateBook 13
[ ] Linux

Additional context

Printing version: printing: ^5.2.1

I already checked the Windows ffi implementation but might take a while for me to fix this issue. If you have any suggestions or insights, I'd be glad to read/hear from you so I can make a PR.

DavBfr / dart_pdf