anchor.diff:    bool get isPossiblyOptional => _isPossiblyOptional;
   
     String get _debugPrefix;
  +
  +  _NullabilityState get _state;
   
     /// Records the fact that an invocation was made to a function with named
     /// parameters, and the named parameter associated with this node was not
  
positive.code: /// Nearly all nullability nodes derive from this class; the only exceptions are
  /// the fixed nodes [NullabilityNode.always] and [NullabilityNode.never].
  abstract class NullabilityNodeMutable extends NullabilityNode {
    bool _isNullable = false;
  
    bool _hasNonNullIntent = false;
  
    /// List of [_NullabilityEdge] objects describing this node's relationship to
    /// other nodes that are "downstream" from it (meaning that if a key node is
  
negative.code:   NullabilityNodeMutable._() : super._();
  
    @override
    bool get isNullable => _state.isNullable;
  }
  
  /// Data structure to keep track of the relationship from one [NullabilityNode]
  