Insertion Sort[Dart]

#### This issue is about [Algo/DS Name](link to resource for the Algo/DS).

- [x] Issue Title is in form `Algo/DS Name [Language]`.
- [x] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [x] This Algo/DS is worth adding to this repository.
- [x] Assign this issue to me (I want to work on this).
- [x] I ensure that I am not already assigned to 2 or more issues.
- [x] I will send my PR only after I'm assigned to this issue by any maintainer.
- [x] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

The issue requests an Insertion Sort implementation in Dart but names no file, test, or entry point. First inspect the repository's existing algorithm implementations and Dart conventions; done means a Dart implementation is added consistently with them.

Written by the indexing model from the issue text.