Leave a comment on the issue, asking to be assigned.
The first person to respond will be assigned. For example:

Once you are assigned, set up your environment, make the change,
and submit the pull request. If you’re not sure how to do this,
see the Quickstart guide below.

The CONTRIBUTING
doc has details on contributing to Flutter.

Discord

Join the #AdoptAWidget channel on the Flutter Discord server
to ask questions and get help.

Guidelines

Wait to be assigned

If you aren’t assigned to the issue, let the assignee fix it.
Don’t submit a pull request for someone else’s issue.

Respond within three days after being assigned

If we don’t hear back from you,
we might need to assign it to someone else.

One issue at a time