title: Is there any way to debug a TypeError: Invalid value used as weak map key?
body: <p>I'm using Dart + Polymer and we were creating a new version of our app, that was going with huge refactoring, deleting some pages and so on. </p>
  
  <p>Now, there is a error  </p>
  
  <blockquote>
    <p>TypeError: Invalid value used as weak map key </p>
  </blockquote>
  
  <p>I assume it is somehow connected to Iron List receiving null data, or some other polymer elements missing something. </p>
  
  <p>But there is no valuable information in stack trace. Is there any way to debug this error, so I can understand the origin of this error?</p>