anchor.diff:  
     /// Return true if this parameter is explicitly marked as being covariant.
     bool get isExplicitlyCovariant {
  -    if (linkedNode != null) {
  -      return linkedContext!.isExplicitlyCovariant(linkedNode!);
  -    }
       return hasModifier(Modifier.COVARIANT);
     }
   
  
positive.code: 
    /// Initialize a newly created variable element to have the given [name] and
    /// [offset].
    VariableElementImpl(String? name, int offset, {Reference? reference})
        : super(name, offset, reference: reference);
  
    VariableElementImpl.forLinkedNode(
        ElementImpl enclosing, Reference? reference, AstNode linkedNode)
        : super.forLinkedNode(enclosing, reference, linkedNode);
  
    /// Initialize using the given serialized information.
    VariableElementImpl.forSerialized(ElementImpl enclosingElement)
        : super.forSerialized(enclosingElement);
  
    /// If this element represents a constant variable, and it has an initializer,
    /// a copy of the initializer for the constant.  Otherwise `null`.
  
negative.code: 
    @override
    String? get defaultValueCode {
      return _defaultValueCode;
    }
  
  