DOCSP-37239: Upgrade to Dart v2.0.0

[cbullinger]

Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-37239

• Flutter SDK: Regenerate snippets with .realm.dart part files, and update part directives to point to new file extension
• Test suite: Upgrade to Dart SDK v2.0.0

Reminder Checklist

Before merging your PR, make sure to check a few things.

• [n/a] Did you tag pages appropriately?
  • genre
  • meta.keywords
  • meta.description
• [x] Describe your PR's changes in the Release Notes section
• [x] Create a Jira ticket for related docs-app-services work, if any

Release Notes

• Flutter SDK
  • Update Bluehawked code snippets throughout docs to use the new .realm.dart part file extension.

Review Guidelines

REVIEWING.md

[docs-builder-bot]

✨ Staging URL: https://preview-mongodbmongodb.gatsbyjs.io/realm/master/

🪵 Logs

• job log: ee0c65b82f4b830bce4ed03600350f9eb577f080