Steps:
1. Create an object
2. Add Rigidbody2D to this object.
3. Add custom script in which mass of the object is changed to negative value
gameObject.rigidbody.mass = -1;

When you play the scene there will be no error or warning messages.
For Rigidbody in the same situation you'll get an error:
Body::setMass: mass is -1.000000, should be positive!
UnityEngine.Rigidbody:set_mass(Single)