code: function () {
              var doc = this.document;
              try {
                  return !doc ? null : browser.ie9below ? doc.selection : domUtils.getWindow( doc ).getSelection();
              } catch ( e ) {
                  return null;
              }
          }
docs: 获取原生seleciton对象
  @method getNative
  @return { Object } 获得selection对象
  @example
  ```javascript
  editor.selection.getNative();
  ```