bug: the appInstance variable is shared by all the instances of the web component

[humanchimp]

Hello. The appInstance variable is shared by all the instances of the web component in a manner that is subject to race conditions. i recommend storing it as an instance property on the class instead. This bug exhibits when multiple instances will contend for that variable, and then they'll end up dispatching, say webComponentAttributeChanged, to the wrong instance. I hope someone finds this insight helpful! Sorry I didn't have time to make a PR. Thanks!