[google_maps_flutter] Add marker clustering support - web implementation

[jokerttu]

This PR introduces support for marker clustering for Web platform

This is prequel PR for: #4319
and sequel PR for: #6158

Containing only changes to google_maps_flutter_web package.

Follow up PR will hold the app-facing plugin implementation.

Linked issue: flutter/flutter#26863

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki 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.
• 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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[jokerttu]

@ditman
Web does not have example app to verify the clustering functionality.

Instead use app-facing package from this PR to test the clustering support on web platform: #4319
And use dependency overriding to force platform implementations from PR:s

To clone and start run the app-facing example app with web clustering from this PR, follow these steps:

1. Clone and open git@github.com:flutter/packages.git
2. Fetch pull request ref: git fetch origin pull/4319/head
3. Create local branch from fetch head: git checkout -b clustering-pr-4319 FETCH_HEAD
4. Add following dependency overrides:

dependency_overrides:
google_maps_flutter_android:
git:
url: git@github.com:flutter/packages.git
path: packages/google_maps_flutter/google_maps_flutter_android
ref: refs/pull/6185/head
google_maps_flutter_ios:
git:
url: git@github.com:flutter/packages.git
path: packages/google_maps_flutter/google_maps_flutter_ios
ref: refs/pull/6186/head
google_maps_flutter_web:
git:
url: git@github.com:flutter/packages.git
path: packages/google_maps_flutter/google_maps_flutter_web
ref: refs/pull/6187/head

5. Start example on using web platform at /packages/google_maps_flutter/google_maps_flutter/example

[jokerttu]

@ditman is it possible to get review for this PR as iOS implementation is already reviewed and Android has only minor test related tasks to do.

[ditman]

Seems to work on my machine!

[ditman]

The marker clustering is super intrusive with the markers themselves, but I guess that's the way it was done in the SDK.

Most of my comments are nitpicking, but there's one blocking observation: we can't add package:js to the dependencies. I hope the links I left help migrating the js-interop layer of markerclusterer to something that is wasm-friendly.

Thanks for the PR and the patience!

[jokerttu]

@ditman@srujzs
Should I remove these lines and update the SDK constraints to 3.3.1 within this PR, or should this be updated separately?
See: CodemateLtd#6 (comment)

[ditman]

I think we can clean them up all together later.

[ditman]

Giving this a final test, want to see if the plugin still works without adding the clustering JS dependency (obviously NOT on the clustering screens, but elsewhere :))

[ditman]

I still believe marker_clustering_js_interop.dart should be a separate package, and that it should inject the JS as needed, but for now this is good.

I tested without the JS in the page, and everything works until you attempt to "Add cluster manager", and then it fails with a fairly explicit Uncaught TypeError: Cannot read properties of undefined (reading 'MarkerClusterer'), which is fine.

Let's ship this one!