Properties outside the properties object

From @hadriann on February 5, 2018 17:31

Description

The usage of properties outside of the properties object is not documented. From the docs:

In most cases, a property that's part of your element's public API should be declared in the properties object.

It is implied that there are use cases when you need not declare a property in the properties object.

One such use case might be a component's private property (like a config), that is set by the component itself (not received via attributes), and passed down to one of its children.

There are a few questions regarding this:

1. Is the above scenario (or any scenario) a valid use case for outside-the-properties properties?
2. Will basic data binding work for such properties?
3. Will computed data binding work?
4. Will such a property be bound at the first render (ready) of the markup?
5. What is the best practice (Polymer-way) to declare or use such private properties?

Copied from original issue: Polymer/polymer#5093