anchor.diff:  /// 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;
  +  _NullabilityState _state = _NullabilityState.undetermined;
   
     /// List of [_NullabilityEdge] objects describing this node's relationship to
     /// other nodes that are "downstream" from it (meaning that if a key node is
  
positive.code:             continue nextEdge;
            }
          }
          if (node is NullabilityNodeMutable && node._becomeNullable()) {
            // Was not previously nullable, so we need to propagate.
            pendingEdges.addAll(node._downstreamEdges);
            if (node is NullabilityNodeForSubstitution) {
  
negative.code:   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
  