title: How to resolve "Attributes on element-name were data bound prior to Polymer upgrading the element?
body: <p>Since the latest version of Polymer (0.2.4) I get the following warning for a bunch of elements:</p>
  
  <blockquote>
    <p>Attributes on us-requests-list were data bound prior to Polymer upgrading the element. This may result in incorrect binding types.</p>
  </blockquote>
  
  <p>How do I resolve this warning? I've found the following issue on Github:</p>
  
  <p><a href="https://github.com/Polymer/polymer-dev/issues/20">https://github.com/Polymer/polymer-dev/issues/20</a></p>
  
  <p>Where they say: </p>
  
  <blockquote>
    <p>Bindings made to an element before it is upgraded can end up being
    attribute bindings where property bindings were expected.</p>
    
    <p>Generally this happens when a user has loaded element definitions
    out-of-order.</p>
  </blockquote>
  
  <p>But what does that mean? For me the warning happens for elements that don't have any bindings or attributes at all.</p>
  
  <p>Perhaps someone can explain to me what these words mean so I can be more specific about my question. It seems to me like the warnings happen at random, the only thing I can say about them is that they only happen on custom elements that are nested in other custom elements, but I don't think it happens for all my nested custom elements.</p>