circular reference when JSON.stringify an element with attached event listener

[sod]

zone.js might break code where an object has an HTMLElement inside and another code tries to JSON.stringify it.

Without zone.js

HTMLElement is serialized to {} by JSON.stringify:

With zone.js 0.8.5 and an event attached to the element

[JiaLiPassion]

@sod, thank you for posting the issue, I will make a PR to fix this one.