anchor.diff:          _dependenciesByAllowed
               .putIfAbsent(fact.allowed.toRef(), () => new Set())
               .add(fact);
  +      } else if (fact is Incompatibility) {
  +        fact = _incompatibilityIntoIncompatibilities(fact);
  +        if (fact == null) continue;
         }
   
         _toProcess.addAll(_fromCurrent);
  
positive.code:     }
    }
  
    // Resolves [required] and [disallowed], which should refer to the same
    // package.
    //
  
negative.code:   }
  
    void _removeDependency(Dependency dependency);
  
    void _removeIncompatibility(Incompatibility incompatibility);
  
    /// If [dependencies]' dependers cover all of [depender], returns the union of
    /// their allowed constraints.
  