[pigeon] Write FlutterError utilities for HostAPI

[Fernthedev]

It seems that when you have a pigeon configuration as follows, some C++ Utility classes are not written in the pigeon header which are required for the generated methods.

import 'package:pigeon/pigeon.dart';
@ConfigurePigeon(PigeonOptions(
  dartOut: 'lib/src/pigeon.g.dart',
  dartOptions: DartOptions(),
  cppOptions: CppOptions(namespace: "pigeon", ),
  cppHeaderOut: 'windows/runner/pigeon.g.h',
  cppSourceOut: 'windows/runner/pigeon.g.cpp',
  objcHeaderOut: 'macos/Runner/pigeon.g.h',
  objcSourceOut: 'macos/Runner/pigeon.g.m',
  // Set this to a unique prefix for your plugin or application, per Objective-C naming conventions.
  objcOptions: ObjcOptions(prefix: 'PGN'),
  // copyrightHeader: 'pigeons/copyright.txt',
  dartPackageName: 'desktop_adb_file_browser',
))

@FlutterApi()
abstract class Native2Flutter {
  void onClick(bool forward);
}

// For pigeon bug-testing
// Uncomment to generate FlutterError types 
// @HostApi()
// abstract class Flutter2Native {
//   void random(bool x);
// }

I'm not sure why this condition existed in the first place, as it was generating since v10.0.0. Regardless, it breaks compiles on Windows. The HostAPI methods still require FlutterError as shown below, so this clearly is necessary.

// Generated class from Pigeon that represents Flutter messages that can be called from C++.
class Native2Flutter {
 public:
  Native2Flutter(flutter::BinaryMessenger* binary_messenger);
  static const flutter::StandardMessageCodec& GetCodec();
  void OnClick(
    bool forward,
    std::function<void(void)>&& on_success,
    std::function<void(const FlutterError&)>&& on_error);

 private:
  flutter::BinaryMessenger* binary_messenger_;
};

List which issues are fixed by this PR. You must list at least one issue.

• Generate FlutterError and other utilities in the C++ header as previously done (and currently required).

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

A few things to mention:

• I was unable to sign the CLA, apparently some server issue or something?
• Running flutter test in the packages/pigeon directory just spammed a lot of dart:mirror not supported errors.
• I didn't file a bug report for this issue as noted in the Tree Hygiene wiki page, but I can do so if needed (again, small fix so I didn't bother)

Hopefully not a problem, but feel free to cherry pick or just manually implement the fix if these issues block the merge.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie or stuartmorgan on the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[tarrinneal]

This will also need tests added to the swift_generator_tests.dart file testing that a file with only flutter api does generate the ErrorOr class.

If you don't want to deal with these changes, I can wrap it up into my pr that's about to land. let me know.

Thanks for catching this.

[tarrinneal]

the conditional isn't needed here. Just put _writeErrorOr above the pre-existing conditionals.

[tarrinneal]

This will need to be it's own version bump. With associated changes required (changelog, pubspec, and generator_tools)

[Fernthedev]

If you have a PR about to land then feel free to supersede this PR. Appreciate the work!

P.S apologies for the messy issue, I was in a rush.

Good luck!

[tarrinneal]

Closing as fixed in #5355