IMap is just a red herring.
The issue, it seems, is that some part of the build process is fetching the types in "haxe.Constraints", and is including "Constraints" in its result.
This problem has precedence– for instance, here's an issue from a year ago where your problem manifested in Haxe's auto-completion functionality:
I wouldn't be surprised if you'll require the help of OpenFL's maintainers. They're usually quick to address these issues! :)