[pigeon] Use cascade operator in the dart_generator.dart when generate Dart code to avoid cascade_invocations warning

[EchoEllet]

Use cascade operator in the dart_generator.dart when generate Dart code to avoid cascade_invocations warning.

A quick fix to this comment (should discuss the fix before attempt to), maybe we should ignore the warning instead at the top of the file in ignore_for_file or provide the option to ignore them all instead.

Also maybe we can introduce some tests to ensure no warning failures for the generated code and that all of them are ignored or solved.

• Related Flutter #155512

Also should have this change in CHANGELOG.md

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene 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 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 this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• 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.

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[EchoEllet]

Closing the issue, see #7725