anchor.diff:    var index = teacherselector.selectedIndex;
     classesselector.options[index].selected = true;
     chooseclassTeacher = classesselector.options[index].firstChild.nodeValue;
  -
   }
   
   void Modeshift(MouseEvent event){               ///转换到标签模式
  
positive.code:       teacherselector.children.add(option1);
        print(classList[1][j]+"done");
      }
    } else {
      querySelector('#Myclassbt').text='nanguo';
    }
  
negative.code: String USERID;             //用于记录用户名（一直可以使用）
  var chooseclassTeacher,chooseclassCourse;           //用于记录选择的
  List classList = new List();
  SelectElement classesselector = new SelectElement();
  SelectElement teacherselector = new SelectElement();
  
  