title: Property validation in Polymer
body: <p>In a <em>normal</em> JavaScript class I can use constructor and setters to ensure that my object is never passed invalid values and thus never enters an invalid state.</p>
  
  <p>How would I do this in Polymer?</p>
  
  <p>Here is why I am asking. I have a bug resulting from one of my component properties having an invalid value. I'm not sure how it became invalid. Normally, my first step would be to ensure that it's not being passed bad values. I would do this by using constructors and setters. </p>
  
  <p>With Polymer being a layer of magic on top of Plain Old JavaScript, I'm not sure how to approach this problem.</p>