code: function(){
              var start = this.startContainer;
              if(this.collapsed && start.nodeType == 3
                  && start.nodeValue.replace(new RegExp('^' + domUtils.fillChar),'').length + 1 == start.nodeValue.length
                  ){
                  return true;
              }
              return false;
          }
docs: 判断当前选区内容是否占位符
  @private
  @method inFillChar
  @return { Boolean } 如果是占位符返回true，否则返回false