$wnd.HTMLImports is undefined when using Vaadin GWT Polymer Elements

I tried to use the Polymer widgets from Vaadin. However, no matter what I do I keep getting the error below basically saying that $wnd.HTMLImports is undefined. I even created a bare-bones project, just with the necessary code to run an example and I still get this error. Any idea what's causing it? 

thank you

tony

Caused by: com.google.gwt.core.client.JavaScriptException: (TypeError) : Cannot read property 'whenReady' of undefined 

TypeError: Cannot read property 'whenReady' of undefined

    at lQf_g$ (Polymer.java:364)

    at QPf_g$ (Polymer.java:277)

    at TQf_g$ (PolymerWidget.java:17)

    at u1f_g$ (PaperMaterial.java:39)

    at K$f_g$ (JavaApiWidget.java:17)

the error is thrown in com.vaadin.polymer.Polymer.java on line 364 which is this:

        $wnd.HTMLImports.whenReady(!e ? f : function() {

  /**

     * Executes a function after all imports have been loaded and when the

     * passed element is ready to use.

     */

    public static native void whenReady(Function f, Element e)

    /*-{

        $wnd.HTMLImports.whenReady(!e ? f : function() {

          var id = setInterval(function() {

            if (@com.vaadin.polymer.Polymer::isRegisteredElement(*)(e)) {

              clearInterval(id);

              f(e);

            }

          }, 0);

        });

    }-*/;

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