[flutter_lints] remove prefer_const_constructors, prefer_const_declarations, prefer_const_literals_to_create_immutables

[devoncarew]

This PR removes three const lints from package:flutter_lints: prefer_const_constructors, prefer_const_declarations, and prefer_const_literals_to_create_immutables.

This PR does not rev the pubspec version. We want to stage this change out in coordination with package:lints; see dart-lang/lints#209.

• Remove the annoying const lints from flutter_lints dart-lang/lints#205

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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[devoncarew]

I believe this PR is except from the changelog check, as we're intentionally staging this change in prep for a publish once the next major version of package:lints is available.

[devoncarew]

cc @Piinks and @stuartmorgan as likely reviewers (normally I would ask @goderbauer for a review here / he would author the PR).