anchor.diff:    EdgeOriginKind get kind => EdgeOriginKind.nonNullAssertion;
   }
   
  -/// Edge origin resulting from the presence of a call to quiver's
  -/// `checkNotNull`.
  -///
  -/// For example, in the following code snippet:
  -///   import 'package:quiver/check.dart';
  -///   void f(int i) {
  -///     checkNotNull(i);
  -///   }
  -///
  -/// this class is used for the edge connecting the type of f's `i` parameter to
  -/// `never`, due to the `checkNotNull` call proclaiming that `i` is not `null`.
  -class QuiverCheckNotNullOrigin extends EdgeOrigin {
  -  QuiverCheckNotNullOrigin(Source source, SimpleIdentifier node)
  -      : super(source, node);
  -
  -  @override
  -  String get description => 'value checked to be non-null';
  -
  -  @override
  -  EdgeOriginKind get kind => EdgeOriginKind.quiverCheckNotNull;
  -}
  -
  -/// Edge origin resulting from the presence of a call to
  -/// `ArgumentError.checkNotNull`.
  -///
  -/// For example, in the following code snippet:
  -///   void f(int i) {
  -///     ArgumentError.checkNotNull(i);
  -///   }
  -///
  -/// this class is used for the edge connecting the type of f's `i` parameter to
  -/// `never`, due to the `checkNotNull` call proclaiming that `i` is not `null`.
  -class ArgumentErrorCheckNotNullOrigin extends EdgeOrigin {
  -  ArgumentErrorCheckNotNullOrigin(Source source, SimpleIdentifier node)
  -      : super(source, node);
  -
  -  @override
  -  String get description => 'value checked to be non-null';
  -
  -  @override
  -  EdgeOriginKind get kind => EdgeOriginKind.argumentErrorCheckNotNull;
  -}
  -
   /// Edge origin resulting from the presence of an explicit nullability hint
   /// comment.
   ///
  
positive.code: 
    @override
    EdgeOriginKind get kind => EdgeOriginKind.parameterInheritance;
  }
  
  /// Edge origin resulting from an inheritance relationship between two method
  
negative.code:   EdgeOriginKind get kind => EdgeOriginKind.optionalFormalParameter;
  }
  
  /// Edge origin resulting from an inheritance relationship between two method
  /// parameters.
  class ParameterInheritanceOrigin extends EdgeOrigin {
  