anchor.diff:  
     @override
     void _preVisit(AssignedVariables<Node, Var> assignedVariables) {
  -    statements._preVisit(assignedVariables);
  -  }
  -
  -  @override
  -  void _visit(
  -      Harness h, FlowAnalysis<Node, Statement, Expression, Var, Type> flow) {
  -    statements._visit(h, flow);
  +    for (var statement in statements) {
  +      statement._preVisit(assignedVariables);
  +    }
  +  }
  +
  +  @override
  +  void _visit(
  +      Harness h, FlowAnalysis<Node, Statement, Expression, Var, Type> flow) {
  +    for (var statement in statements) {
  +      statement._visit(h, flow);
  +    }
     }
   }
   
  
positive.code:   final Statement? initializer;
    final Expression? condition;
    final Expression? updater;
    final List<Statement> body;
    final bool forCollection;
  
    _For(this.initializer, this.condition, this.updater, this.body,
  
negative.code: /// to create instances of this class.
  class SwitchCase implements _Visitable<void> {
    final bool _hasLabel;
    final _Block _body;
  
    SwitchCase._(this._hasLabel, this._body);
  
    String toString() => [
          if (_hasLabel) '<label>:',
          'case <value>:',
          ..._body.statements
        ].join(' ');
  
    void _preVisit(AssignedVariables<Node, Var> assignedVariables) {
      _body._preVisit(assignedVariables);
  