   Returns the first index of <code class="prettyprint lang-dart">element</code> in this list.
              </dd>
              <dt id="insert" class="callable">
                  <span class="name ">insert</span><span class="signature">(<wbr><span class="parameter" id="insert-param-index"><span class="type-annotation">int</span> <span class="parameter-name">index</span></span>, <span class="parameter" id="insert-param-element"><span class="type-annotation"><a href="polymer_app_layout/Plugin-class.html">Pl