Object.observe（） 观察对象
这个对象方法可以用来异步观察对javascript对象的改动：
// Let's say we have a model with data
var model = {};
// Which we then observe
Object.observe(model, function(changes) {
// This asynchronous callback runs
changes.forEach(function(change) {
// Letting us know what changed
console.log(change.type, change.name, change.oldValue);
});
});
