For Google3, this makes it easier since we only need to make one source transform (from `#include "flutter` -> `#include "third_party` instead of requiring a source transform for every root directory in this repo. *List which issues are fixed by this PR. You must list at least one issue.* *If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].* [C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style