Zone是一个执行上下文，能够在多个异步任务之间持久化。可以将其类比成为JavaScript VM设计的thread-local storage.

A Zone is an execution context that persists across async tasks. You can think of it as thread-local storage for JavaScript VMs.

So, without zones, we don’t get any change detection, so we don’t get any of the nice UI updates that we’d expect!

没有zone-evergreen就没有change detection.