[BUG] Unknown identifier Assets for argument 'assetHandler'
What does polymod mean by "Assets"?

When loading a script, a class which extends HScriptable will call Assets.getText() to retrieve the data of the script file.
This means that the class needs to import an Assets class, for example lime.utils.Assets or openfl.utils.Assets. HScriptable doesn't add that import for you because we don't know what framework you're using.
If you're having this issue with a specific framework that isn't Lime/OpenFL/HaxeFlixel, let me know.
Leaving this open because this error is supposed to be more verbose (Class does not import an Assets class for Polymod to fetch scripts with!) but it's not for some reason.
Okay, so I did the right thing by importing openfl.utils.Assets.