Add missing void in function declarations to avoid warning

[kristofb]

at build time: "This function declaration is not a prototype"

*Building an application which includes dart_api.h produces warnings with XCode:

This function declaration is not a prototype

Only three functions prototypes are missing (void).
.*

[mraleph]

/cc @athomas @sortie Copybara does not seem to pick this one up?

[sortie]

Copybara is working to avoid the duplicate pull request events. Perhaps their attempts lost this pull request. I'll let them know about it so they can take a look. Otherwise this pull request looks ready with CLA.

@kristofb You can try to push (or force push) to your branch to trigger our copybara automation that imports this pull request into gerrit as a code review. Development of Dart is done on the https://dart-review.googlesource.com gerrit so the pull request must be imported before we can merge your changes. :)

[copybara-service]

Thank you for your contribution! This project uses Gerrit for code reviews. Your pull request has automatically been converted into a code review at:

https://dart-review.googlesource.com/c/sdk/+/307880

• Please wait for a developer to review your code review at the above link (you can speed up the review if you sign into Gerrit and manually add a reviewer that has recently worked on the relevant code). - See CONTRIBUTING.md to learn how to upload changes to Gerrit directly. - Additional commits pushed to this PR will update both the PR and the corresponding Gerrit CL. - After the review is complete on the CL, your reviewer will merge the CL (automatically closing this PR).

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/307880 has been updated with the latest commits from this pull request.