anchor.diff:  Statement for_(Statement? initializer, Expression? condition,
           Expression? updater, List<Statement> body,
           {bool forCollection = false}) =>
  -    new _For(initializer, condition, updater, body, forCollection);
  +    new _For(initializer, condition, updater, block(body), forCollection);
   
   /// Creates a "for each" statement where the identifier being assigned to by the
   /// iteration is not a local variable.
  
positive.code:     }
      _body._preVisit(assignedVariables);
      assignedVariables.endNode(_bodyNode);
      _catches._preVisit(assignedVariables);
      if (_finally != null) {
        if (_catches.isNotEmpty) {
          assignedVariables.endNode(this);
  
negative.code:     new _Break(branchTargetPlaceholder);
  
  SwitchCase case_(List<Statement> body, {bool hasLabel = false}) =>
      SwitchCase._(hasLabel, new _Block(body));
  
  /// Creates a pseudo-statement whose function is to verify that flow analysis
  /// considers [variable]'s assigned state to be [expectedAssignedState].
  