update dart docs to current dart (3.13)

# Bug report
the currently available dart docs are for dart 2 which is over a year old...

## OS information

NixOS
## Steps to reproduce
open up the documentation on devdocs.io for dart and see that it's for dart 2.

## Possible fix

update the docs

Start at the Dart documentation entry on devdocs.io and locate the repository source or configuration that provides its Dart 2 content. Compare the available content with Dart 3.13 documentation and update the relevant source; done means DevDocs presents current Dart 3.13 documentation instead of Dart 2.

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