title: Polymer custom element attributes as boolean typed values
body: <p>While <code>Demo A</code> works, I would like to understand if it is possible to make it work like <code>Demo B</code>. Basically, to for the custom element to resolve <code>happyFlag</code> attribute as boolean type.</p>
  
  <pre><code>// Demo A - works. bob smiles. cat frowns.
  &lt;x-smiley name="Bob" happyFlag="true"&gt;&lt;/x-smiley&gt;
  &lt;x-smiley name="Cat" happyFlag="false"&gt;&lt;/x-smiley&gt;
  &lt;polymer-element name="x-smiley" attributes="name, happyFlag" noscript&gt;
      &lt;template&gt;
          {{name}} is
          &lt;span hidden?="{{happyFlag == 'true'}}"&gt;:-)&lt;/span&gt;
          &lt;span hidden?="{{happyFlag == 'false'}}"&gt;:-(&lt;/span&gt;
          &lt;hr&gt;
      &lt;/template&gt;
  &lt;/polymer-element&gt;
  
  // Demo B - does not work (span always hidden)
  &lt;x-smiley name="Bob" happyFlag="true"&gt;&lt;/x-smiley&gt;
  &lt;x-smiley name="Cat" happyFlag="false"&gt;&lt;/x-smiley&gt;
  &lt;polymer-element name="x-smiley" attributes="name, happyFlag" noscript&gt;
      &lt;template&gt;
          {{name}} is
          &lt;span hidden?="{{happyFlag}}"&gt;:-)&lt;/span&gt;
          &lt;span hidden?="{{happyFlag}}"&gt;:-(&lt;/span&gt;
          &lt;hr&gt;
      &lt;/template&gt;
  &lt;/polymer-element&gt;
  </code></pre>