Consider making shady dom distribution trigger custom elements attached callbacks

[sorvell]

The iron-list element uses Polymer.dom(this).appendChild to attach items. It then needs to measure the item. Since Polymer.dom is asynchronous, it needs to either call flush or measure asynchronously. However, when the custom elements polyfill is used, after the item is asynchronously attached to dom, async to that any custom elements in the item fire their attached callback. This makes it difficult for iron-list to know when it can measure.

If Polymer.dom's rendering process also called CustomElements.takeRecords() then elements like iron-list would have a clear way to know when they can measure rendered items: call this.async or Polymer.dom.flush().