title: Dart Removing disallowed attribute after editor upgraded
body: <p>I have encounter error "Removing disallowed attribute" after I upgraded my dart editor in SDK 0.7.3.1_r27487.</p>
  
  <p>I have a custom tag which template contains boostarp attributes "data-target" and "data-toggle". It work under previous version but encounter error after upgraded.</p>
  
  <p>Console</p>
  
  <pre><code>Removing disallowed attribute &lt;A data-toggle="dropdown"&gt;
  Removing disallowed attribute &lt;BUTTON data-target=".navbar-collapse"&gt;
  Removing disallowed attribute &lt;BUTTON data-toggle="collapse"&gt;
  </code></pre>
  
  <p>.html Code</p>
  
  <pre class="lang-dart prettyprint-override"><code>&lt;element extends="div" name="x-navbar" constructor="Navbar"&gt;
  &lt;template&gt;    
    ..
    &lt;a name="top" href="#" class="dropdown-toggle" data-toggle="dropdown"&gt;Shop &lt;b class="caret"&gt;&lt;/b&gt;&lt;/a&gt;
    ..
    &lt;button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".navbar-collapse"&gt;&lt;/button&gt;
  &lt;/template&gt;
  &lt;script type="application/dart" src="custom_ui.dart"&gt;&lt;/script&gt; 
  &lt;/element&gt;
  </code></pre>