基于原生XMLHttpRequest()封装get方法

待进一步的完善：

function get(url,callback){
            let http = new XMLHttpRequest();
            http.onload = function(){
                callback(http.responseText);
            }
            http.open("get",url,true);
            http.send();
        }
get ("data.json",function(data){
            console.log(data);
 })