Extensions to type extension custom elements must specify an extends property when registering

[sorvell]

Example:

XButtonProto = Object.create(HTMLButtonElement.prototype);
XSubButtonProto = Object.create(XButtonProto);

document.register('x-button', {prototype: XButtonProto, extends: 'button'});
/* extends here is required! */
document.register('x-button', {prototype: XSubButtonProto, extends: 'button'});

[sorvell]

polymer-element currently doesn't handle this case properly.