anchor.diff:      );
     }
   
  +  /// Removes a [variable] from a "promoted" [map], treating the map as
  +  /// immutable.
     Map<Variable, Type> _removePromoted(
         Map<Variable, Type> map, Variable variable) {
       if (map.isEmpty) return const {};
  
positive.code:     this.promoted,
    );
  
    /// Add a new [variable] to track definite assignment.
    State<Variable, Type> add(Variable variable, {bool assigned: false}) {
      var newNotAssigned = assigned ? notAssigned : notAssigned.add(variable);
  
  
negative.code:     );
    }
  
    /// Updates the state to indicate that the control flow path is unreachable.
    State<Variable, Type> exit() {
      return State<Variable, Type>._(
        false,
  