Make class concrete post conditions not working

[seyyedaliayati]

Describe the bug
This refactoring does not check post conditions and after running this on my test project, I got this error when compiling:
The abstract method sides in type "Shape" can only be defined by an abstract classJava(67109227)

To Reproduce
Steps to reproduce the behavior:

1. Go to 'refactorings/make_class_concrete.py'
2. Run the script on your test project
3. Compile the test project
4. See error

Expected behavior
Check post conditions properly. Source code must be compile-able after refactoring!