title: Toggle disabled attribute of paper-button
body: <p>I have a <code>paper-button</code> (send button) which is related to multiple <code>paper-input</code> fields/element.
  The <code>paper-input</code> button should be disabled as long as the user fills out all related <code>paper-input</code> fields/elements (<strong>newEmail</strong> and <strong>passwd</strong>).</p>
  
  <p>This is the important part of the element</p>
  
  <pre><code>  &lt;gold-email-input id="newEmail"
                      label="Email"
                      no-label-float required&gt;
    &lt;/gold-email-input&gt;
  
    &lt;paper-input id="passwd"
                 type="password"
                 label="Password"
                 no-label-float required&gt;
    &lt;/paper-input&gt;
  
    &lt;div class="buttons"&gt;
      &lt;paper-button dialog-dismiss&gt;Cancel&lt;/paper-button&gt;
      &lt;paper-button dialog-confirm autofocus
                    disabled$="[[isInputEmpty($$.newEmail.value, $$.passwd.value)]]"
                    on-tap="changeEmail"
                    class="default"&gt;Save&lt;/paper-button&gt;
    &lt;/div&gt;
  </code></pre>
  
  <p>The function <strong>disabled$="[[isInputEmpty($$.newEmail.value, $$.passwd.value)]]"</strong> won't be called when the newEmail.value or passdw.value change.</p>
  
  <p>This is the JavaScript part:</p>
  
  <pre><code>  isInputEmpty (email, pass) {
      if (email.length === 0 || pass.length === 0) return true;
      return false;
    }
  </code></pre>
  
  <p>What's the correct implementation to toggle the attribute <strong>disabled</strong> between <em>true</em> and <em>false</em>?</p>