[firebase_messaging]

[ericmartineau]

Description

The method FcmDartService#start is not implemented on iOS. This produces a PlatformException if configure is called with onBackgroundMessage.

Since this functionality is only implemented on Android, I altered the configure method to only attempt to call FcmDartService#start when the platform is Android

Related Issues

#1695

Checklist

• [ x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x ] If the pull request affects only one plugin, the PR title starts with the name of the plugin in brackets (e.g. [cloud_firestore])
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• [x ] I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• [x ] I read and followed the Flutter Style Guide.
• [x ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• [x ] I updated CHANGELOG.md to add a description of the change.
• [ x] I signed the CLA.
• [ x] I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• [x ] No, this is not a breaking change.

[ericmartineau]

@kroikie I'm a little unclear on how this process works from here. I'm happy to have contributed this
minor change, but hoping that I won't have to continually rebase out CHANGELOG and pubspec version conflicts for every other change merged ahead of mine...