[google_maps_flutter_web] Add Cloud MapStyle color scheme support for web

[martyfuhry]

This PR adds web support for setting Cloud-based map style color schemes in google_maps_flutter_web.

GoogleMap(
  cloudMapId: cloudMapId,
  colorScheme: MapColorScheme.light, // OR .dark, .followSystem
  // ...
);

In mobile platforms, the system brightness is always used to select the correct cloud-style variant (light or dark). Currently in web, the cloud-style variant is always light. With this change, we set colorScheme by default to .followSystem in order to achieve parity with the mobile platforms.

Additionally, we expose the field for web-only use to override the default behavior. I do not believe that the mobile SDKs expose similar methods, so the value is ignored by them.

Fixes https://github.com/flutter/flutter/issues/176445

What this PR adds

• Serialization of Cloud MapStyle color scheme fields to JS MapOptions

Changelog

Updated CHANGELOG.md following repository CHANGELOG style guidelines.

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Pre-Review Checklist

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• [x] I read the [Tree Hygiene] page, which explains my responsibilities.
• [x] I read and followed the [relevant style guides] and ran [the auto-formatter].
• [x] I signed the [CLA].
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [google_maps_flutter_web]
• [x] I linked to at least one issue that this PR fixes in the description above.
• [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, following repository CHANGELOG style.
• [x] I updated/added any relevant documentation.
• [x] I added new tests to check the change, or I am claiming a test exemption because the change is a thin JS interop addition with no new Dart-side logic paths.
• [x] All existing and new tests are passing.

[flutter-dashboard[bot]]

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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[martyfuhry]

I didn't add any tests since this is a thin JS-interop passthrough with no added business logic. Please let me know if this PR is not exempt.

[stuartmorgan-g]

Thanks for the contribution! You’ve checked boxes in the PR checklist above that are not reflected in this PR, so I’m assuming this is a work in progress and am marking it as a Draft. Please review the checklist, updating the PR as appropriate, and when the state of the PR as posted reflects the checklist please feel free to mark it as ready for review.

• [x] I read the [Contributor Guide] and followed the process outlined there for submitting PRs. [...]
• [x] All existing and new tests are passing.

The PR does not follow our process for changing federated plugins, so most of the CI is failing.

• [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, following repository CHANGELOG style.

This PR changes three packages, but only does this for one of them.

[stuartmorgan-g]

I didn't add any tests since this is a thin JS-interop passthrough with no added business logic. Please let me know if this PR is not exempt.

Which of the documented exemption reasons do you believe this falls under?

The entire plugin is a wrapper that does passthrough to the underlying SDK; it's not clear to me what about this part you believe is exempt from testing, or why it wouldn't matter if this passthrough were to break in the future.

[stuartmorgan-g]

completing parity with the existing mobile implementations

Could you elaborate on what you mean here? The issue description sounds like this is a web-only option, but if similar functionality exists for mobile we should be abstracting that rather than adding a web-only parameter.

[martyfuhry]

Thank you for the detailed feedback @stuartmorgan-g! It is very helpful.

The PR does not follow our process for changing federated plugins, so most of the CI is failing.

Thank you for the link! I did not fully understand the federated plugin flow. I'll follow the documented process and update all affected packages accordingly.

This PR changes three packages, but only does this for one of them.

Got it. I will update the other packages pubspec.yaml and CHANGELOG.md, as well.

Could you elaborate on what you mean here? The issue description sounds like this is a web-only option, but if similar functionality exists for mobile we should be abstracting that rather than adding a web-only parameter.

Right, great point. The mobile platforms always uses your default system brightness and applies that automatically to select the brightness-aware cloud-style variant (light or dark). I don't believe the mobile SDKs expose an equivalent parameter, so this is currently a web-only feature of the cloud styling system. To reduce the ambiguity, I could make the colorScheme default to .followSystem in order to unify the three platforms' behavior.

Which of the documented exemption reasons do you believe this falls under?

You are right, this change doesn't fall under any of the documented automatic exemptions. I'll add a small test that covers this passthrough so that future changes do not accidentally regress the issue.

I appreciate your thorough review and guidance. Thanks for helping me with this submission.

[martyfuhry]

I believe the last remaining test failure is the expected one from docs.

I have updated the PR description with the new changes.

I have also added a unit test.

Please let me if there is anything I have done incorrectly or haven't completed. Thank you again for your instruction.

[martyfuhry]

Thank you again @stuartmorgan-g for the thorough review. I have addressed all of your corrections and am resubmitting for review.