[pigeon] Channel name in flutter error.

[tarrinneal]

Adds the channel name in the connection error message on all platforms host and flutter api methods.

Also fixes a bug with error handling for swift flutter api void methods.

Also adds method to all generators to help clean up duplicated code.

fixes flutter/flutter#136277

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.

[stuartmorgan]

Sorry I didn't think about this before: hard-coding all of these strings in the generated code will unnecessarily bloat the size of the binary, because each unique string will end up as data in the compiled output. For all languages, we should be using string interpolation so that we aren't creating N new strings constants.

And while compilers would probably handle de-duping for us if we inline them separately, it would be better not to rely on that and avoid duplicating even separate constants, so ideally we should have the generation output something like (example in Dart, but similar for the other languages):

const String channelName = "big string constant here, once";
final BasicMessageChannel<Object?> channel = BasicMessageChannel<Object?>(
        channelName,
        codec,
        binaryMessenger: _binaryMessenger);
...
      throw PlatformException(
        code: 'channel-error',
        message:
            'Unable to establish connection on channel: "$channelName".',
      );

[tarrinneal]

Sorry I didn't think about this before: hard-coding all of these strings in the generated code will unnecessarily bloat the size of the binary, because each unique string will end up as data in the compiled output. For all languages, we should be using string interpolation so that we aren't creating N new strings constants.

And while compilers would probably handle de-duping for us if we inline them separately, it would be better not to rely on that and avoid duplicating even separate constants, so ideally we should have the generation output something like (example in Dart, but similar for the other languages):

const String channelName = "big string constant here, once";
final BasicMessageChannel<Object?> channel = BasicMessageChannel<Object?>(
        channelName,
        codec,
        binaryMessenger: _binaryMessenger);
...
      throw PlatformException(
        code: 'channel-error',
        message:
            'Unable to establish connection on channel: "$channelName".',
      );

Makes sense, I made sure to do that in the generators themselves, but I didn't even think about it for the generateds.

This is also a general issue with the duplication of code in the generated files. Something that I'm hoping to tackle some day...

[stuartmorgan]

LGTM with minor changes (some optional). If you want me to re-review the C++ helper change let me know.

[stuartmorgan]

This should either be static or in a namespace { ... } (i.e., an anonymous namespace).