Extending paper-item in Polymer Dart - dart

Question

I have been trying to extend the paper-item element from the paper_elements package. I do things as I have done when extending my own elements, but that fails.
<link rel="import" href="../../packages/polymer/polymer.html">
<link rel="import" href="../../packages/paper_elements/paper_item.html">
<polymer-element name="x-item" extends="paper-item">
...
</polymer-element>
#CustomTag('x-item')
class XItem extends PaperItem {
XItem.created() : super.created();
}
I get no error message, and no Polymer Element is initialised, with a blank part of the page where ever I've put a Polymer element.
I try using the same method I've used to extend a builtin HTML element like button or div. That also fails.
<link rel="import" href="../../packages/polymer/polymer.html">
<link rel="import" href="../../packages/paper_elements/paper_item.html">
<polymer-element name="x-item" extends="paper-item">
...
</polymer-element>
#CustomTag('x-item')
class XItem extends PaperItem with Polymer, Observable {
XItem.created() : super.created() {
super.polymerCreated();
}
}
And I get
Breaking on exception: NotSupportedError: Registration failed for type 'x-item'. The tag name specified in 'extends' is a custom element name. Use inheritance instead.
If I remove the extends part of the polymer template definition, again I get nothing. Where the Polymer elements in the page have been placed, there is nothing but blankness.
The previous error still happens if I import the actual JS version polymer-item.html via
<link rel="import" href="../../packages/paper_elements/src/paper-item/paper-item.html">
though I still have to extend PaperItem with Polymer and Observable, using the super.polymerCreated(); to generate that error.
I know that the Polymer Dart Team did some trickery to get the underlying paper-elements, which are written in JS, to work almost like they are Polymer Dart elements. I guess this is what's causing the problems. My question is how do I overcome this, so I can extend a paper-element in Dart?
UPDATE - A bug has been submitted https://code.google.com/p/dart/issues/detail?id=20388

Answer 1

I tried it and it also didn't work for me.
I'm pretty sure this is a bug/missing feature with the Dart wrapper for the Polymer.js elements used in paper-elements/core-elements.
You haven't added the HTML of your element.
Does your extending elements template contain a <shadow></shadow> element?
<polymer-element name="x-item" extends="paper-item">
<template>
<shadow></shadow>
</template>
</polymer-element>