async awit的用法

示例

	function sum(a, b){
		return new Promise((resolve, reject)){
			setTimeout(()=>{
				return a+b;
			}, 100000); // 10s后返回求和结果
		}
	}
   getSumAsync: async funtion(){
   		let res = await sun(1, 2);
   		console.log("sum:"+res);
   }