anchor.diff:  ///   last option that's not disabled, depending on direction, otherwise focus
   ///   goes to the selected (except when disabled, then skips group entirely);
   ///   and jumps out of the group on second tab.
  -///
  -/// __Example usage:__
  -///
  -/// Checked status is set on individual radio, only listen to selection at
  -/// group level.
  -///
  -/// ```html
  -///   <material-radio-group [selectionModel]="mySingleSelectionModel">
  -///     <material-radio [checked]="true"
  -///                     [value]="option0">default choice
  -///     </material-radio>
  -///     <material-radio [checked]="false"
  -///                     [value]="option1">alternative choice
  -///     </material-radio>
  -///   </material-radio-group>
  -/// ```
  -///
  -/// Selection of the value is done at group level, can also be done
  -/// via [ngModel].
  -///
  -/// ```html
  -///   <material-radio-group [(selected)]="selectedOption">
  -///     <material-radio [value]="option0">default choice
  -///     </material-radio>
  -///     <material-radio [value]="option1">alternative choice
  -///     </material-radio>
  -///   </material-radio-group>
  -/// ```
   @Component(
     selector: 'material-radio-group',
     template: '<ng-content></ng-content>',
  
positive.code:   dynamic _preselectedValue;
    bool _isContentInit = false;
  
    /// Value of currently selected radio.
    @Input()
    set selected(dynamic selectedValue) {
      if (_radioComponents != null && _isContentInit) {
  
negative.code:     });
    }
  
    /// Published when selection changes. Prefer `(ngModelChanged)`.
    @Output('selectedChange')
    Stream<dynamic> get onChange => _onChange.stream;
    final _onChange = new StreamController<dynamic>.broadcast();
  