[dart] fix bug which generated wrong code

[fawdlstty]

Because the [#8380] author hasn't updated the working tree for a long time, I decided to reopen [#8259]

[fawdlstty]

@aardappel Hello, can you take some time to review it?

[aardappel]

Like the other PR, you need to include exactly all the changes this has on the test generated code files in this repo.

[fawdlstty]

The test was modified by another author. I don't think I'm suitable to submit. Do you have any suggestions?

[fawdlstty]

@insertjokehere Hello. Do you have time to resubmit your test modifications?

[aardappel]

You just need to run the script. If you look at the CI failures above, you'd see:

ERROR: ********************************************************
ERROR: * The following differences were found after running   *
ERROR: * the script/generate_code.py script. Maybe you forgot *
ERROR: * to run it after making changes in a generator?       *
ERROR: ********************************************************

[fawdlstty]

I noticed that the error message was caused by the generation module of the Rust language and TypeScript language, but this patch did not modify that part. I think this question needs to be fixed in another patch, not here.

[github-actions]

This pull request is stale because it has been open 6 months with no activity. Please comment or label not-stale, or this will be closed in 14 days.