Consider issuing a warning when a polymer-element's shadowRoot contains un-upgraded custom elements

[sorvell]

Polymer attempts to guarantee that custom elements inside another custom element's shadowRoots are upgraded when it is created. This allows the binding system to depend on interacting with upgraded elements. It also allows users to be able to interact with upgraded elements are create time.

Polymer requires that element definitions are registered at upgrade time to be able to enforce this guarantee. As a debugging aide, when this is not the case, polymer should issue a warning that a custom element contains an un-upgraded custom element in its shadowRoot.

[sorvell]

We've decided against this.