anchor.diff:    /// variable that is no longer in scope.
     final Map<Variable, VariableModel<Variable, Type> /*!*/ > variableInfo;
   
  -  /// Variable model for variables that have never been seen before.
  -  final VariableModel<Variable, Type> _freshVariableInfo;
  -
     /// The empty map, used to [join] variables.
     final Map<Variable, VariableModel<Variable, Type>> _emptyVariableMap = {};
   
  
positive.code: 
    /// Creates a [VariableModel] representing a variable that's never been seen
    /// before.
    VariableModel.fresh()
        : promotedTypes = null,
          tested = const [],
          assigned = false,
          unassigned = true,
          writeCaptured = false;
  
    /// Returns a new [VariableModel] in which any promotions present have been
  
negative.code:         );
  
    @visibleForTesting
    FlowModel.withInfo(this.reachable, this.variableInfo) {
      assert(reachable != null);
      assert(() {
        for (VariableModel<Variable, Type> value in variableInfo.values) {
  