http://www.w3.org/TR/2013/WD-components-intro-20130606/ (the latest version) lists the document.register() signature for extensions with three arguments:

var FancyButton = document.register('button', 'fancy-button', {prototype: p});

However in the spec at https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/custom/index.html#dfn-type-extension it's listed with two arguments:

document.register('x-foo', {
    prototype: Object.create(HTMLParagraphElement.prototype, {
        firstMember: {
            get: function() { return "foo"; },
            enumerable: true,
            configurable: true
        },
        // specify more members for your prototype.
        // ...
    })
});


Since Polymer also works with two arguments, I'm guessing the example in the into is wrong.