iterate objects polymer. not work.

index.js

  // paste into console on http://www.polymer-project.org/ and inspect document body
  // template appears, but not rendered data
  
  var doc = document.implementation.createHTMLDocument()
  doc.body.innerHTML = [
    '<template repeat="{{key, value in items}}">',
    '  {{key}} {{value}}',
    '</template>'
  ].join('\n')
  
  var t = doc.body.firstChild
  t.bindingDelegate = new PolymerExpressions()
  t.model = {items: {
    a: 1,
    b: 2,
    c: 3,
    d: 4 
  }}
  
  document.body.appendChild(t)