anchor.diff:  ///           child: const Text('Label One'),
   ///         ),
   ///         new Semantics(
  -///           sortOrder: new SemanticsSortOrder(key: new OrdinalSortKey(2.0)),
  +///           sortOrder: new SemanticsSortOrder(key: new OrdinalSortKey(1.0)),
   ///           child: const Text('Label Two'),
   ///         ),
   ///       ],
  
positive.code: /// The [OrdinalSortKey] compares itself with other [OrdinalSortKey]s
  /// to sort based on the order it is given.
  ///
  /// The ordinal value `order` is typically an integer, though it can be
  /// fractional, e.g. in order to fit between two other consecutive integers. The
  /// value must be finite (it cannot be a NaN value or infinity).
  ///
  /// See also:
  ///
  ///  * [SemanticsSortOrder] which manages a list of sort keys.
  class OrdinalSortKey extends SemanticsSortKey {
    /// Creates a semantics sort key that uses a double as its key value.
    ///
    /// The [order] must be a finite number.
    const OrdinalSortKey(
      this.order, {
      String name,
    }) : assert(order != null),
         assert(order > double.NEGATIVE_INFINITY),
         assert(order < double.INFINITY),
         super(name: name);
  
    /// A double which describes the order in which this node is traversed by the
    /// platform's accessibility services. Lower values will be traversed first.
    final double order;
  
    @override
  
negative.code: 
    /// The implementation of [compareTo].
    ///
    /// The argument is guaranteed to be of the same type as this object.
    ///
    /// The method should return a negative number if this object comes earlier in
    /// the sort order than the argument; and a positive number if it comes later
    /// in the sort order. Returning zero causes the system to default to
    /// comparing the geometry of the nodes.
  