Accessing App Constants doesn't seem to be that easy

I have a constant calcMeasurement which is a list. I have some custom code that creates a drop down list whose list values will be populated by that constant's values.

I am stalling at the import of the package. Flutter predefines the path the app_constants.dart package but it will not compile due the package not existing. Seems counter intuitive to give me a predefined path that does not exist.

Please explain if I am doing something wrong.