anchor.diff:        ActiveItemModel activeModel,
         int charCode,
         SelectionOptions options,
  -      ItemRenderer itemRenderer,
  +      ItemRenderer<T> itemRenderer,
         SelectionModel selection) {
       // Guard against being called when not all data is initialized.
       if (itemRenderer == null || options == null) return;
  
positive.code: 
  // TODO(google): Move it to a common home to increase reusability.
  // TODO(google): Better comparison of characters to better support i18n.
  class ActivateItemOnKeyPressMixin {
    static Map<int, String> _charCodeMap = <int, String>{};
  
    String _enteredKeys = '';
  
negative.code:         MaterialButtonWrapper,
          TrackLayoutChangesMixin,
          KeyboardHandlerMixin,
          ActivateItemOnKeyPressMixin<T>,
          ShiftClickSelectionMixin<T>
      implements PopupSizeProvider, OnChanges, OnDestroy {
    /// Function for use by NgFor for optionGroup.
  