Incorrect type definition for template.host

[davidturissini]

Description

https://github.com/salesforce/lwc/blob/master/packages/%40lwc/engine/types/engine.d.ts has incorrect typings for template.host

Steps to Reproduce

Create a new typescript project with LWC. Try to access this.template.host in connectedCallback

Expected:
this.template.host should be cast as either null | HTMLElement

Actual:
this.template.host is always null: https://github.com/salesforce/lwc/blob/master/packages/%40lwc/engine/types/engine.d.ts#L88

[caridy]

It can never be null since this is a hard reference to the host that you can disconnect.