anchor.diff:      }());
     }
   
  +  String toString() {
  +    StringBuffer sb = new StringBuffer();
  +    sb.write('AssignedVariables(');
  +    _printOn(sb);
  +    sb.write(')');
  +    return sb.toString();
  +  }
  +
     /// This method should be called during pre-traversal, to mark a write to a
     /// variable.
     void write(Variable variable) {
  
positive.code: 
    bool isTracked(Node node) => _writtenInNode.containsKey(node);
  
    Set<Variable> writtenInNode(Node node) => _getWrittenInNode(node);
  
    @override
    void _printOn(StringBuffer sb) {
      super._printOn(sb);
      sb.write(',_declaredInNode=$_declaredInNode');
    }
  
    String toString() {
      StringBuffer sb = new StringBuffer();
      sb.write('AssignedVariablesForTesting(');
      _printOn(sb);
      sb.write(')');
      return sb.toString();
    }
  }
  
  
negative.code:     sb.write('_writtenStack=$_writtenStack,');
      sb.write('_capturedStack=$_capturedStack');
    }
  }
  
  /// Extension of [AssignedVariables] intended for use in tests.  This class
  