anchor.diff:  class NgNullForm implements NgForm {
     var _name, _dirty, _valid, _invalid, _pristine, _element;
     var _controls, _scope, _parentForm, _controlName;
  -  var currentErrors, _controlByName;
  +  var errors, _controlByName;
     dom.Element element;
     NgNullForm() {}
     operator[](name) {}
  
positive.code:   final dom.Element _element;
    final Scope _scope;
  
    final Map<String, List<NgControl>> currentErrors =
        new Map<String, List<NgControl>>();
  
    final List<NgControl> _controls = new List<NgControl>();
  
negative.code:    * error is real).
     */
    setValidity(NgControl control, String errorType, bool isValid) {
      List queue = errors[errorType];
  
      if(isValid) {
        if(queue != null) {
          queue.remove(control);
          if(queue.isEmpty) {
            errors.remove(errorType);
            if(errors.isEmpty) valid = true;
            _parentForm.setValidity(this, errorType, true);
          }
        }
      } else {
        if(queue == null) {
          queue = new List<NgControl>();
          errors[errorType] = queue;
          _parentForm.setValidity(this, errorType, false);
        } else if(queue.contains(control)) return;
  
  