this.node.webkitShadowRoot needs to return ShadowRoot #68
Description
Originally from: https://github.com/toolkitchen/polyfills/issues/82
When using <script shadow="shim">, accessing this.node.webkitShadowRoot in the custom element should return a reference it's ShadowRoot.
https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#shadow-aware-attributes