Fixes for static

[tristanMatthias]

I noticed that you were referencing properties and listeners to the element/target. These should actually be referenced via (target.constructor as Element).properties, as they are static variables in Polymer.

class Test extends PolymerElement {
    static get properties() {...}
}

As a result, I moved them over to the target.constructor which now works in Polymer@^3.0.2

There is also a few tidyups with typings.