[multicast_dns] MDnsClient::listen supports onError callback

[alexander-irion]

MDnsClient::Listen now supports an optional onError callback function,
called in case of a stream error. If omitted any errors on the stream
are considered unhandled, and will be passed to the current [Zone]'s
error handler. By default unhandled async errors are treated as if they
were uncaught top-level errors.

This fixes an unhandled exception occuring (tested on Android), when the
network is disconnected.

Issue: flutter/flutter#165482

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I signed the CLA.
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• I linked to 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 I have commented below to indicate which version change exemption this PR falls under¹.
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If you need help, consider asking for advice on the #hackers-new channel on Discord.

How can a test be added? It would need to disconnect the device from WLAN beforehand.

Footnotes

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[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

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?

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[stuartmorgan]

Thanks for the submission!

In the future, please do not delete the checklist that is in the PR template; it is there for a reason. This PR is missing required elements described in the checklist (I’ve restored it to the PR description), which need to be addressed before it moves forward with review.

I am marking the PR as a Draft. Please review the checklist, updating the PR as appropriate, and when that’s complete please feel free to mark the PR as ready for review.

[alexander-irion]

@stuartmorgan : How can a test be added? It would need to disconnect the device from WLAN beforehand.

[alexander-irion]

@vashworth: Beside a missing test, would the change make sense for you?

[vashworth]

Thanks for adding this! This makes sense to me.

[alexander-irion]

@vashworth: Hi, what's missing for a merge?