Im not sure if this is the basic cause of the problem but to be able to import a package the package must have something inside its lib
directory.
You should move your main.dart
into the subfolder bin
of your dart_demo
package if it is a command line application or web
if its code for a web page.
You should move your core.dart
file into the subfolder lib
of your core
package.
If it still doesn't work pleas add a comment.