Containment associations can't be created for abstract classes in patterns

1. Create a metamodel with a class Abs and a class Con.
2. Make Abs abstract, and have a containment relation from Con to Abs
3. Create CS and pattern metamodels
4. Load the pattern metamodel as a toolbar
5. Create a Con and an Abs element
6. Drag the Abs element inside the Con element.
7. Observe as either the console reports that a containment relationship can't be created, or AToMPM crashes.

Reproduce the failure using the seven-step metamodel, toolbar, and drag-and-drop sequence described in the issue, then inspect the containment-association handling reached by that interaction. Done means an abstract Abs element can be dragged into a Con element without a console error or crash.

Written by the indexing model from the issue text.