Description

There has been some confusion about whether or not
.flutter-plugins-dependencies should be tracked in version control or
not. Added a comment to both it and .flutter-plugins explaining that
it's generated and shouldn't be.

.flutter-plugins-dependencies is parsed through JSON, and the JSON spec
doesn't support comments. So unfortunately the note is living in an
arbitrary "_info" key instead of an obvious top level comment.

Related Issues

#45188

Tests

I added the following tests:

• Modified packages/flutter_tools/test/general.shard/plugins_test.dart to check for the new file content

Checklist

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• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
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Breaking Change

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• No, this is not a breaking change.