Custom properties fail to resolve when an element is attached to an unknown shadowRoot

[sorvell]

Given an element added to an unknown shadowRoot like this:

var root = unknownHostElement.createShadowRoot();
root.appendChild(document.createElement('x-foo'));

An exception will occur due to the fact that it's assumed that the dom host of the x-foo element has a known styling api which can be interrogated. In this case the dom host of x-foo is not a Polymer element. We'll need to make the property resolver smarter about finding a styling host to avoid this problem and ensure that elements in unknown hosts are properly styled.