三种不同创建示例的方式
   var people = new People();
   var people = People();
   var people = const People();

class  People {
	String name;  //默认 null
	um age;   // 默认 null
}
创建类示例
var people = new People();
people.name = "zhangsan";
people.age = 12;
在类中定义的非final属性，默认生成set,get方法。比如people.name相当于people.setName("zhangsan");
people.name 相当于 people.getName();