[google_maps_flutter] Add marker clustering support - web implementation

[jokerttu]

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This PR introduces support for marker clustering for Web platform

This is prequel PR for: https://github.com/flutter/packages/pull/4319 and sequel PR for: https://github.com/flutter/packages/pull/6158

Containing only changes to google_maps_flutter_web package.

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

Linked issue: https://github.com/flutter/flutter/issues/26863

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] 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.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [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, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[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: https://github.com/flutter/packages/pull/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 [email protected]: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: [email protected]:flutter/packages.git
      path: packages/google_maps_flutter/google_maps_flutter_android
      ref: refs/pull/6185/head
  google_maps_flutter_ios:
    git:
      url: [email protected]:flutter/packages.git
      path: packages/google_maps_flutter/google_maps_flutter_ios
      ref: refs/pull/6186/head
  google_maps_flutter_web:
    git:
      url: [email protected]: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]

@jokerttu I've created a fix for the JS-interop layer and left it on your repo:

• https://github.com/CodemateLtd/packages/pull/6

[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 :))